Malware comes in five main types: viruses, worms, Trojans, spyware, and adware. Viruses replicate and spread between computers, worms transfer without user interaction, Trojans disguise as useful programs but compromise security, spyware secretly monitors user activity, and adware displays unwanted advertisements. Additional internet threats include spam emails, phishing for personal details, botnets of compromised computers, and denial of service attacks that disrupt access to services.