An annuity is an insurance product; annuities are typically issued by the same companies that issue life insurance policies, and the risks undertaken by the issuer are fundamentally the same for both products -- that is, the insurance company bets on the life expectancy of the customer. The result is to transfer the effects of the uncertainty of an individual's lifespan from the individual to the insurer, which reduces its own uncertainty by pooling many clients.

With a "single premium" or "immediate" annuity, the annuitant pays for the annuity with a single lump sum. The annuity starts making regular payments to the annuitant within a year. A common use of a single premium annuity is as a destination for roll-over retirement savings upon retirement. In such a case, a retiree withdraws all of the money the retiree has saved in, for example, a 401(k) (i.e., tax-advantaged) savings vehicle during the retiree's working life and uses the money to buy an annuity whose payments will replace the retiree's wage payments for the rest of the retiree's life. The advantage of such an annuity is that the annuitant has a guaranteed income for life, whereas if the retiree were instead to withdraw money regularly from the retirement account, the retiree might run out of money before the retiree dies or not have as much to spend while the retiree is alive.

Another kind of annuity is a combination of retirement savings and retirement payment plan: the annuitant makes regular contributions to the annuity until a certain date and then receives regular payments from the annuity until the annuitant dies. Sometimes there is a life insurance component added so that if the annuitant dies before annuity payments begin, a beneficiary gets either a lump sum or annuity payments.

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 the annuity phase in which the insurance company makes income payments until the death of the customers (the "annuitants") named in the contract. It is possible to structure an annuity contract so that it has only the annuity phase; such a contract is called an immediate annuity. Annuity contracts with a deferral phase are similar to bank CDs and have a growth phase prior to distribution of income, and are called deferred annuities. The newest incarnation is the fixed, equity indexed product which can be either a fixed annuity or pure life insurance.

Such contracts provide an income during retirement or a stream of payments as a settlement of a personal injury lawsuit (i.e., a structured settlement). Some annuities (called "joint life" or "joint and survivor" annuities) continue paying a second person (i.e., the "beneficiary") after the annuitant dies, until that person dies as well. (For example, an annuity may be structured to make payments to a married couple, such payments ceasing on the death of the second spouse.)

Annuities that make payments in fixed amounts or in amounts that increase by a fixed percentage are called fixed annuities. Variable annuities, by contrast, pay amounts that vary according to the investment performance of a specified set of investments, typically bond and equity mutual funds.

Variable annuities are used for many different objectives. One common objective is deferral of the recognition of taxable gains. Money deposited in a variable annuity grows on a tax-deferred basis, so that taxes on investment gains are not due until a withdrawal is made. Variable annuities offer a variety of funds ("subaccounts" in the parlance of the industry) from various money managers. This gives investors the ability to move between subaccounts without incurring additional fees or sales charges.