Malware

Attaches itself to a legitimate program and spreads when that program is executed. Spreads computer-to-computer (usually via internet or removable media). It can lie dormant until triggered by a specific date or event.

Disguises itself as legitimate software to trick the user into downloading it. Once installed, the creator gains remote control of the infected computer. Unlike viruses and worms, Trojans do NOT self-replicate.

Exploits security holes in networks to spread automatically. Unlike a virus, it does NOT attach to other files — it is a standalone program. Its main damage is consuming disk space and slowing network bandwidth as it replicates continuously.

Silently collects user data — browsing habits, usernames, passwords, credit card details — and sends it to a hacker. A keylogger is a specific type that records every keystroke. Does NOT replicate.

Downloaded with free software, usually without the user's knowledge. Generates unwanted advertisements, often targeted to the user's browsing habits. Generally more of a nuisance than dangerous, but often linked with spyware in modern attacks.

Installs a set of tools that give the attacker continuous, privileged (administrator-level) remote access. It hides deep within the OS — the user has no idea it's there. It can even disable or remove anti-virus software.

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