A virus is a malicious piece of software programmed to spread itself over the Internet.
Viruses can range from fairly harmless to ones that can cause data loss.
You should always have a current virus protection program on your computer. It should also protect you from worms and trojan horses. A Trojan horse is what seems to be legitimate software, but does something destructive when run. It is likely to be a hoax. Spreading hoaxes damages the reputation of both the Internet and the companies involved.