What is malware and what are the three main types?

Malware is malicious code/software; the three main types are viruses, worms, and Trojan horses.

Malware (malicious software) is code designed to damage, disrupt, steal, or inflict illegitimate action on data, hosts, or networks.

* The three classic types differ in how they spread: a virus rides a host program, a worm spreads itself, a Trojan tricks the user. *

Three main types:

1. Viruses - Executable files that propagate by inserting copies into other programs; require a host program to spread

2. Worms - Standalone programs that spread through networks without requiring a host program

3. Trojan horses - Malware that users are tricked into installing; creates backdoors for attackers; does NOT self-replicate

Go deeper:

• What is the difference between viruses, worms, and Trojan horses? — DigiCert — concise side-by-side of the three behaviours.
• Malware — Wikipedia — broader taxonomy placing virus/worm/Trojan among ransomware, spyware, and other sub-types.