Viruses - A virus is a
software program that is capable of replicating with little or no user intervention, and the replicated programs also replicate further. Viruses piggyback on real programs. For example, a virus may attach itself to a program such as a spreadsheet program. Each time the spreadsheet program runs, the virus runs as well, and it has the opportunity to reproduce by attaching to other programs. The purpose of a virus can be anything from erasing files, formatting your hard drive, to replacing text in your document. Viruses are often disguised as games or images with clever titles such as "Pictures of ME".
E-mail Viruses - An email virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to the people in the victim’s e-mail address book.
Worms- A worm is a virus that spreads by creating duplicates of itself on other drives, systems, or networks. Worms may send copies of themselves to other computers across network connections, through e-mails, through infected web pages, or through instant messages.
Trojan Horses - This program may claim to do one thing (such as claiming to be a game) but instead does damage when you run it. Trojan horses are not technically viruses, since they do not replicate.