What is the difference between a virus and a worm?

A virus is a program that is designed to spread from file to file on a single PC, it does not intentionally try to move to another PC, and it must replicate, and execute itself to be defined as a virus.

A worm is designed to copy itself (intentionally move) from PC to PC, via networks, internet etc.

A worm doesnt need a host file to move from system to system, whereas a virus does.

So worms spread more rapidly then viruses.

The Word 'Virus' has become a common term a lot of people use to refer to worms and trojans too. Which is not exactly correct.