Traditionally, annuities are contracts from insurance companies that offer a stream of income in return for your premium paid. The income stream can be guaranteed to last as long as you live or it can be guaranteed for a period of time such as for 20 years. Should you live longer, the annuity continues paying out for the remainder of your life. Other variations exist.
Note that some people that own annuities never actually "annuitize" their policy to start the income stream. Instead, they use these policies for their wealth accumulation properties and safety of their capital. Be sure to read the Buyer's Guide.
ALWAYS CHECK THE SPECIFICS OF YOUR OWN ANNUITY CONTRACT(S)
There are three main "categories" that define annuity contract types. The frequency of premium payments, the annuity payout timeline, and the interest earning characteristic of the annuity can all be used to define the type of annuity contract.
Single Premium Annuity
Flexible Premium Annuity
Scheduled Premium Annuity
For more detailed information, we highly recommend reading the booklet from the National Association of Insurance Commissioners-