In many ways, HIV is just like any other virus. All viruses consist of genetic material surrounded by a protein coat termed the capsid. Some, like HIV, also have an additional protective membrane called the envelope. Viruses multiply by infecting cells of the host organism and making copies of themselves, a process called viral replication. Importantly, HIV is a retrovirus.
What is remarkable about retroviruses? The cells that form our body and also many viruses make proteins by a precise sequence of molecular events; the genetic material called DNA is used as a template to make another molecule termed RNA in a process called transcription; RNA is then used as a template to synthesise a specific protein. This is the central dogma of molecular biology which put simply, states that "DNA makes RNA makes protein".
Retroviruses are different in that their genetic material is RNA. Once inside a newly infected cell, viral RNA genetic material has to be copied into viral DNA, before more viral RNA and ultimately viral proteins are produced. So, in the case of a retrovirus such as HIV, it is more accurate to say "RNA makes DNA makes RNA makes protein". The synthesis of HIV’s DNA from RNA is called reverse transcription (as it is in the opposite direction to normal transcription or "DNA makes RNA") and is mediated by a viral enzyme called reverse transcriptase. The activity of reverse transcriptase is crucial for the successful infection of cells by HIV, a feature that has been exploited to develop drugs.