Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
A retrovirus carries its genome as RNA rather than DNA (DNA being the "ordinary" carrier for a genetic code). One of the proteins encoded by the retroviral genome is an unusual enzyme called reverse transcriptase, which transcribes RNA into DNA. With this enzyme HIV can create viral DNA when it enters a cell, and the viral DNA can then go on to be integrated into the host genome.
Reverse transcriptase is an unusual enzyme because its function is not needed by most organisms - human cells don't have their own reverse transcriptase enzymes, for example. This makes reverse transcriptase inhibitors a prime idea for HIV therapy!