Malware (malicious software) is any software designed to damage, disrupt, or gain unauthorized access to a system. This lesson covers the key differences between common malware types: viruses (attach to files and spread when executed), worms (self-replicate and spread across networks without user action), and trojans (disguise themselves as legitimate software to trick users into installing them). Understanding these distinctions helps in recognizing how different threats spread and what kind of defenses are effective against each.