A retrovirus is a virus whose genes are encoded in RNA instead of DNA. Although, like other viruses, retroviruses need to use the cellular machinery of the organisms they infect to make copies of themselves, infection by a retrovirus requires an additional step. The retrovirus genome needs to be reverse-transcribed into DNA by an enzyme called reverse-transcriptase before it can be copied in the usual way.
Retroviruses use reverse-transcriptase to transform their single-stranded RNA into double-stranded DNA so that it can be integrated into the genome of the cells they have infected. Once the DNA version of the retroviral genes have been incorporated into a cell's genome, the cell copies those genes as part of its normal replication process. Thus, the cell does the work of the virus for it.
The most well known retrovirus that infects humans is HIV. However, there are several other human retroviruses, including the human T-cell lymphotropic virus 1 (HTLV-1), which is associated with certain T-cell leukemias and lymphomas. There are many additional retroviruses which have been identified as infecting other species.