Annuities are contractually-executed, relatively low-risk investment products; as the insured (usually, an individual) pays a life insurance company a lump-sum premium at the start of the contract.  That money is to be paid back to the insured in fixed, incremental amounts, over some future time period (predetermined by the insured).  The insurer invests the premium; the resulting profit/return on investment fund the payments received by the insured, and, compensate the insurer.  Conventional annuity contracts provide a predictable, guaranteed stream of future income (e.g., for retirement) until the death(s) of the beneficiaries(s) named in the contract, or, until a future termination date – whichever occurs first.

Annuity contracts in the United States are defined by the Internal Revenue Code and regulated by the individual states.  Variable annuities have features of both life insurance and investment products.  In the U.S., annuity insurance may be issued only by life insurance companies, although private annuity contracts may be arranged between donors to non-profits to reduce taxes. Insurance companies are regulated by the states, so contracts or options that may be available in some states may not be available in others. Their federal tax treatment, however, is governed by the Internal Revenue Code. Variable annuities are regulated by the Securities and Exchange Commission and the sale of variable annuities is overseen by the Financial Industry Regulatory Authority (FINRA) (the largest non-governmental regulator for all securities firms doing business in the United States).

There are two possible phases for an annuity, one phase in which the customer deposits and accumulates money into an account (the deferral phase), and another phase in which customers receive payments for some period of time (the annuity or income phase). During this latter phase, the insurance company makes income payments that may be set for a stated period of time, such as five years, or continue until the death of the customer(s) (the "annuitant(s)") named in the contract. Annuitization over a lifetime can have a death benefit guarantee over a certain period of time, such as ten years. Annuity contracts with a deferral phase always have an annuity phase and are called deferred annuities. An annuity contract may also be structured so that it has only the annuity phase; such a contract is called an immediate annuity. Note this is not always the case.

The term "annuity," as used in financial theory, is most closely related to what is today called an immediate annuity. This is an insurance policy which, in exchange for a sum of money, guarantees that the issuer will make a series of payments. These payments may be either level or increasing periodic payments for a fixed term of years or until the ending of a life or two lives, or even whichever is longer. It is also possible to structure the payments under an immediate annuity so that they vary with the performance of a specified set of investments, usually bond and equity mutual funds. Such a contract is called a variable immediate annuity. See also life annuity, below.

The overarching characteristic of the immediate annuity is that it is a vehicle for distributing savings with a tax-deferred growth factor. A common use for an immediate annuity might be to provide a pension income. In the U.S., the tax treatment of a non-qualified immediate annuity is that every payment is a combination of a return of principal (which part is not taxed) and income (which is taxed at ordinary income rates, not capital gain rates). Immediate annuities funded as an IRA do not have any tax advantages, but typically the distribution satisfies the IRS RMD requirement and may satisfy the RMD requirement for other IRA accounts of the owner (see IRS Sec 1.401(a)(9)-6.)

When a deferred annuity is annuitized, it works like an immediate annuity from that point on, but with a lower cost basis and thus more of the payment is taxed.

Annuity with period certain.  This type of immediate annuity pays the annuitant for a designated number of years (i.e., a period certain) and is used to fund a need that will end when the period is up (for example, it might be used to fund the premiums for a term life insurance policy). Thus this option is not necessarily suitable for an individual's retirement income, as the person may outlive the number of years the annuity will pay.

A life or lifetime immediate annuity is used to provide an income for the life of the annuitant similar to a defined benefit or pension plan.

A life annuity works somewhat like a loan that is made by the purchaser (contract owner) to the issuing (insurance) company, which pays back the original capital or principal (which isn't taxed) with interest and/or gains (which is taxed as ordinary income) to the annuitant on whose life the annuity is based. The assumed period of the loan is based on the life expectancy of the annuitant. In order to guarantee that the income continues for life, the insurance company relies on a concept called cross-subsidy or the "law of large numbers". Because an annuity population can be expected to have a distribution of lifespans around the population's mean (average) age, those dying earlier will give up income to support those living longer whose money would otherwise run out. Thus it is a form of longevity insurance (see also below).

A life annuity, ideally, can reduce the "problem" faced by a person when they don't know how long they will live, and so they don't know the optimal speed at which to spend their savings. Life annuities with payments indexed to the Consumer Price Index might be an acceptable solution to this problem, but there is only a thin market for them in North America.

The second usage for the term annuity came into being during the 1970s. Such a contract is more properly referred to as a deferred annuity and is chiefly a vehicle for accumulating savings with a view to eventually distributing them either in the manner of an immediate annuity or as a lump-sum payment.

All varieties of deferred annuities owned by individuals have one thing in common: any increase in account values is not taxed until those gains are withdrawn. This is also known as tax-deferred growth.

A deferred annuity which grows by interest rate earnings alone is called a fixed deferred annuity (FA). A deferred annuity that permits allocations to stock or bond funds and for which the account value is not guaranteed to stay above the initial amount invested is called a variable annuity (VA).

A new category of deferred annuity, called the fixed indexed annuity (FIA) emerged in 1995 (originally called an Equity-Indexed Annuity).  Fixed indexed annuities may have features of both fixed and variable deferred annuities. The insurance company typically guarantees a minimum return for EIA. An investor can still lose money if he or she cancels (or surrenders) the policy early, before a "break even" period. An oversimplified expression of a typical EIA's rate of return might be that it is equal to a stated "participation rate" multiplied by a target stock market index's performance excluding dividends. Interest rate caps or an administrative fee may be applicable.

Deferred annuities in the United States have the advantage that taxation of all capital gains and ordinary income is deferred until withdrawn. In theory, such tax-deferred compounding allows more money to be put to work while the savings are accumulating, leading to higher returns. A disadvantage, however, is that when amounts held under a deferred annuity are withdrawn or inherited, the interest/gains are immediately taxed as ordinary income.

A variety of features and guarantees have been developed by insurance companies in order to make annuity products more attractive. These include death and living benefit options, extra credit options, account guarantees, spousal continuation benefits, reduced contingent deferred sales charges (or surrender charges), and various combinations thereof. Each feature or benefit added to a contract will typically be accompanied by an additional expense either directly (billed to client) or indirectly (inside product).

Deferred annuities are usually divided into two different kinds:

Fixed annuities offer some sort of guaranteed rate of return over the life of the contract. In general such contracts are often positioned to be somewhat like bank CDs and offer a rate of return competitive with those of CDs of similar time frames. Many fixed annuities, however, do not have a fixed rate of return over the life of the contract, offering instead a guaranteed minimum rate and a first year introductory rate. The rate after the first year is often an amount that may be set at the insurance company's discretion subject, however, to the minimum amount (typically 3%). There are usually some provisions in the contract to allow a percentage of the interest and/or principal to be withdrawn early and without penalty (usually the interest earned in a 12-month period or 10%), unlike most CDs. Fixed annuities normally become fully liquid depending on the surrender schedule or upon the owner's death.

Most equity index annuities are properly categorized as fixed annuities and their performance is typically tied to a stock market index (usually the S&P 500 or the Dow Jones Industrial Average). These products are guaranteed but are not as easy to understand as standard fixed annuities as there are usually caps, spreads, margins, and crediting methods that can reduce returns. These products also don't pay any of the participating market indices' dividends; the trade-off is that contract holder can never earn less than 0% in a negative year.

Variable annuities allow money to be invested in insurance company "separate accounts" (which are sometimes referred to as "subaccounts" and in any case are functionally similar to mutual funds) in a tax-deferred manner. Their primary use is to allow an investor to engage in tax-deferred investing for retirement in amounts greater than permitted by individual retirement or 401(k) plans. In addition, many variable annuity contracts offer a guaranteed minimum rate of return (either for a future withdrawal and/or in the case of the owner's death), even if the underlying separate account investments perform poorly. This can be attractive to people uncomfortable investing in the equity markets without the guarantees. Of course, an investor will pay for each benefit provided by a variable annuity, since insurance companies must charge a premium to cover the insurance guarantees of such benefits. Variable annuities are regulated both by the individual states (as insurance products) and by the Securities and Exchange Commission (as securities under the federal securities laws). The SEC requires that all of the charges under variable annuities be described in great detail in the prospectus that is offered to each variable annuity customer. Of course, potential customers should review these charges carefully, just as one would in purchasing mutual fund shares. People who sell variable annuities are usually regulated by FINRA, whose rules of conduct require a careful analysis of the suitability of variable annuities (and other securities products) to those to whom they recommend such products. These products are often criticized as being sold to the wrong persons, who could have done better investing in a more suitable alternative, since the commissions paid under this product are often high relative to other investment products.

Deferred annuities are generally sold by financial professionals, some of whom may work directly for an insurance company. Most financial professionals, however, are independent agents of the insurance company, not employees.

Variable annuities are controversial because many believe the extra fees (i.e., the fees above and beyond those charged for similar retail mutual funds that offer no principal protection or guarantees of any kind) may reduce the rate of return compared to what the investor could make by investing directly in similar investments outside of the variable annuity. A big selling point for variable annuities is the guarantees many have, such as the guarantee that the customer will not lose his or her principal. Critics say that these guarantees are not necessary because over the long term the market has always been positive, while others say that with the uncertainty of the financial markets many investors simply will not invest without guarantees.[who?] Past returns are no guarantee of future performance, of course, and different investors have different risk tolerances, different investment horizons, different family situations, and so on. The sale of any security product should involve a careful analysis of the suitability of the product for a given individual.

In the U.S. Internal Revenue Code, the growth of the annuity value during the accumulation phase is tax-deferred, that is, not subject to current income tax, for annuities owned by individuals. The tax deferred status of deferred annuities has led to their common usage in the United States. Under the U.S. tax code, the benefits from annuity contracts do not always have to be taken in the form of a fixed stream of payments (annuitization), and many annuity contracts are bought primarily for the tax benefits rather than to receive a fixed stream of income. If an annuity is used in a qualified pension plan or an IRA funding vehicle, then 100% of the annuity payment is taxable as current income upon distribution (because the taxpayer has no tax basis in any of the money in the annuity). If the annuity contract is purchased with after-tax dollars, then the contract holder upon annuitization recovers his basis pro-rata in the ratio of basis divided by the expected value, according to the tax regulation Section 1.72-5. (This is commonly referred to as the exclusion ratio.) After the taxpayer has recovered all of his basis, then 100% of the payments thereafter are subject to ordinary income tax.