Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
Both DNA and RNA are composed of long chains of sugar residues (held together with phosphates), each sugar having a base attached to it:
Because the sugar residues are not symmetrical (at one point they're joined at the "fifth" position on their carbon chain, and at the other they're joined at the "third" position) we can distinguish the two ends of a DNA or RNA chain from each other - the 5' end and the 3' end.
Deoxyribose Nucleic Acid, DNA, consists of two long chains of nucleotides twisted into a double helix. The sugar is deoxyribose. The two chains are joined by hydrogen bonding between complementary bases on the nucleotide chains: adenine (A) and thymine (T) hydrogen will hydrogen-bond together, as will cytosine (C) and guanine (G).
DNA encodes the genome of most organisms. However, its close chemical relation RNA (Ribose Nucleic Acid - the sugar residue is ribose) is what HIV uses to encode the HIV genome. A free HIV particle holds its genome on two RNA strands - but when the virus is integrated into its host cell, its genetic information is encoded as DNA (otherwise it couldn't be part of the host DNA).
"Transcription" is the process of copying a DNA code into RNA - and so "reverse transcription" is how we refer to copying an RNA code into DNA.