Americans hear a lot about the shaky outlook for Social Security. In the future, the federal program likely will play a smaller overall role in Americans' retirement plans.

One way to fill in the gaps of a savings portfolio is to put money in annuities. With an annuity, you pay a premium in exchange for guaranteed income payments at regular intervals. It is most often used for retirement purposes.

The basic types of annuities are equity indexed, fixed rate and variable. The major advantage of annuities is that they all guarantee benefits such as tax-free growth, the ability to pass money directly to heirs or charities and an income stream for life.

Over the past few years, equity-indexed annuities have gained a great deal of popularity. They offer interest or benefits that are linked to an external equity reference - a stock index like the S&P 500, for example. But you get a guaranteed minimum return in exchange for a limited maximum return; that is, you get less upside, but also less downside, to your stock-market investing. Your principal is never at risk.

Fixed-rate annuities, on the other hand, guarantee an interest rate and a declared minimum. They have traditionally been the most popular annuities.

Variable annuities provide more options. They enable you to invest in stock, bonds, mutual funds and money-market instruments.

Reputable financial companies, like TrueYield Financial, want to make sure investors are comfortable when purchasing annuities. Here are some tips for the potential investor.

* Be sure the firm you work with is not limited to offering just one company's annuities. There are many options available, so work with an agent that can get the one that best fits your needs.

* Understand what you are buying. Talk to your financial adviser or agent about which annuity may be right for your retirement portfolio. Fully understand the annuity contract you are considering.

* Define your goals. Annuities can be used to accomplish a number of financial goals. For example, they can supplement your monthly income or provide emergency funds. Decide which purpose your annuity will serve.

* Ask your agent if you have a "free look" period to review your annuity contract and make sure you have made the right decision.

* Investigate whether or not a bonus annuity is right for you. Bonus annuities credit premium bonuses to allow a retirement saver to make up for stock market loss or to provide an immediate boost to the account value.