Definitions
What is a Virus?
A virus is a manmade program or piece of code that causes an unexpected, usually negative, event. Viruses are often disguised games or images with clever marketing titles such as "Me, nude."

What is a Worm?
Computer Worms are viruses that reside in the active memory of a computer and duplicate themselves. They may send copies of themselves to other computers, such as through email or Internet Relay Chat (IRC).

What is a Trojan Horse?