What is a retrovirus?

The genes of most living things, including humans, are made of DNA. The DNA exists as a sequence of a code that can be read like a book. In the cell the code is read to make RNA which is then used as the code for the construction of proteins. In other words, the flow of genetic information in the cell is usually from DNA to RNA to protein.
The HIV virus, on the other hand, has its genetic material made from RNA. It has to insert its genetic code into that of the host cell in order to replicate. In order to achieve this it must first make a DNA copy so that it is compatible with the DNA of the host cell. DNA is then made using the code of the RNA. Since this is the opposite of the usual case the viruses that do this are called retroviruses