When HIV infects a cell, reverse transcriptase copies the viral single stranded RNA genome into a double-stranded viral DNA. The viral DNA is then integrated into the host chromosomal DNA, which then allows host cellular processes, such as transcription and translation to reproduce the virus. RTIs block reverse transcriptase's enzymatic function and prevent completion of synthesis of the double-stranded viral DNA, thus preventing HIV from multiplying.
A similar process occurs with other types of viruses. The hepatitis B virus, for example, carries its genetic material in the form of DNA, and employs a RNA-dependent DNA polymerase to replicate. Some of the same compounds used as RTIs can also block HBV replication; when used in this way they are referred to as polymerase inhibitors.
RTIs come in three forms:
Nucleoside analog reverse-transcriptase inhibitors (NARTIs or NRTIs)
Nucleotide analog reverse-transcriptase inhibitors (NtARTIs or NtRTIs)