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.
Read more about this topic: Annuity (US Financial Products)