What's the difference between a worm and a virus

What is the technical difference between a worm and a virus?

As I understand it, a worm actively seeks out new victims (or attempts to send itself to new victims) once it has infected a host. A virus won’t do this, if my understanding is correct.

Actually, a virus replicates itself by inserting itself into other programs, so that when that program is opened the infection spreads.

A worm, on the other hand, is a self-contained program. Once executed, it replicates by sending out copies of itself, but it does not insert itself into other code.

Barry

McAfee Virus Glossary

Symantec’s description of the difference between a worm, virus and trojan.