Although index annuity products are considered to be fixed deferred annuities and feature all the benefits that fixed annuities have to offer, an index annuity is different from other fixed annuities in the way it credits interest. Most fixed annuities only credit interest calculated at a rate set in the contract. Indexed annuities, on the other hand, credit interest using a formula based on changes in the particular index to which the annuity is linked.