What is the difference between a virus, a worm, and a Trojan?

A virus is malicious code that attaches to a legitimate program or file and requires user action to activate, such as opening an infected attachment. It replicates by inserting copies into other programs. A worm is self-replicating malware that spreads automatically across networks without user interaction by exploiting vulnerabilities, consuming bandwidth and resources. A Trojan appears to be legitimate software but contains hidden malicious functionality. Trojans do not self-replicate but rely on social engineering to trick users into installing them.