Viruses & malware come in many forms including computer viruses, worms, Trojan horses, spyware, and adware. Computer viruses can infect systems and replicate without user knowledge. Types of viruses include resident, direct action, overwrite, boot, macro, directory, and polymorphic viruses. Worms replicate to spread to other systems without user intervention. Trojans appear benign but perform malicious actions like unauthorized access. Spyware and adware secretly monitor users or display advertisements.