-A worm is a standalone piece of malicious software that reproduces itself and spreads from computer to computer.
-A virus is a piece of computer code that inserts itself within the code of another standalone program, then forces that program to take malicious action and spread itself.
-A trojan is a program that cannot reproduce itself but masquerades as something the user wants and tricks them into activating it so it can do its damage and spread.
Antivirus software is the most widely known product in the category of malware protection products; despite "virus" being in the name, most offerings take on all forms of malware. While high-end security pros dismiss it as obsolete, it's still the backbone of basic anti-malware defense.