Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
"Tat" is short for "transactivator" - it's a regulatory gene which accelerates the production of more HIV virus. In fact, it's crucial to HIV, because HIV completely fails to replicate itself without it. Tat protein is also toxic, so the large amounts of tat protein released into the blood by HIV-infected cells are no help for the body.
Tat works because the protein encoded by tat binds to the start of a new HIV RNA strand - a part which has been called the "Transactivator Active Region" or TAR. The TAR runs from +1 to +59, that is to say, the first 59 nucleotides of the HIV genome. Once the cellular machinery has transcribed this much provirus into RNA, tat can bind to it and encourage the transcription of the remainder of the HIV genetic code.
You might have also read about the negative regulators which HIV has - NRE, nef, vif. Surely it's barmy to have genes for boosting virus reproduction as well as genes for suppressing it! Well actually this is the normal way of things down at the level of genes and proteins. The tug-of-war between the suppressors and the activators can result in an incredibly precise control of how much a gene is expressed. Without this tug-of-war control, gene expression would simply depend on how active the cell's transcription machinery was - either all the genes would be expressed a lot, or all the genes wouldn't be expressed much at all. Organisms such as cells need more precision than that!