What is a Virus?
A virus is a program or piece of code that has been created to cause an unexpected and usually negative event.

What is a Worm?
A worm is a virus that sits in the active memory of a computer and duplicates itself. It may send copies of themselves to other computers, such as through email or Internet Relay Chat (IRC).

What is a Trojan Horse?
A Trojan horse program is a malicious program that pretends to be a benign application; a Trojan horse program purposefully does something the user does not expect. Trojans are not viruses since they do not replicate, but Trojan horse programs can be just as destructive.

Many people use the term to refer only to non-replicating malicious programs, thus making a distinction between Trojans and Worms.