The most classic kind of malware is a virus, which attaches itself to existing files by making a small modification and changing the file’s behaviour. Viruses are very rare these days, but the term is sometimes used for any kind of malware.

A worm is malware that automatically spreads to other systems. Worms still exist, but they are less common than they once were.

A trojan (or trojan horse) is malware that pretends to be a legitimate program and thus tricks the user into installing or executing it. For example, cracked versions of paid software often include trojans.