Viruses are small sets of genes packed in a protein coat. They depend on cells (e.g. human cells can be infected by certain viruses) to carry out their replication but have a stage in their life cycle that is independent of cells. This independent stage is called the infectious virus particle as it is able to infect a cell (if it encounters a cell of the right type) and enlist that cell in producing multiple copies of the infectious particle. Viruses are classified into broad categories based on their pattern of gene replication and the chemical nature of their genes packed with the virus coat. A large and diverse group of viruses use RNA for storage of genetic information in the infectious virus particle, while many other groups of viruses use DNA for this purpose. Many RNA viruses do not use DNA at any stage of their life cycle. Retroviruses are unusual in that they use RNA to store genetic information in infectious virus particles, but form a version of that information in the form of DNA during their replication cycle. Subsequently in that life-cycle, that DNA is used to provide a template for transcription of RNA that gets packaged in the next generation of viral particles.

Pararetroviruses are DNA viruses – that is, the infectious virus particles of parareroviruses carry DNA, but their genetic information transitions through an RNA strand as part of their life cycle in a manner that is reminiscent of retrovirus replication. Despite their names, retroviruses and pararetroviruses are not closely related. See also RNA.