Michael Oldstone described a virus as "A bit of nucleic acid surrounded by bad news."
For a better (and more scientific) explanation:
A virus is a biological agent that can exist only inside a living cell. It replicates when the cell replicates, or replicates inside the cell. It is not generally considered to be a living thing, despite the presence of genetic material in its make up. There are over 5000 viruses known and a few are damaging to the health of the host.