Difference between trojan , virus, and worm?

What's the difference between trojan , virus, and worm?

A trojan is a destructive app that disguises itself as something else. it is aptly named after the Trojan Horse that Homer wrote about (the Greeks gave the Trojans a 'peace offering' of a large wooden horse, in fact there were soldiers hidden inside and they came out and ambushed the Trojans).

A virus is a program or piece of code that can be highly desctructive or simply annoying. It is usually attached to a program and its 'installation' is initiated by downloading the infected program or file. It can also replicate itself though, like...

A worm. Which is nearly the same as a virus except it it is known for replicating itself and spreading over networks, causing damage much the same way as a virus would.

Yeah, worms go into your contact list and email themselves to everyone on them. Pretty ingenious, but a HUGE headache.:o