What is malware?

Malware is a broad category of unwanted software that intentionally harms computer systems or networks.  Common examples include viruses, trojans, rootkits, worms, and ransomware.  Malware will directly interact with computers, often attempting to either overwrite files, replicate itself and retransmit to other devices, or even compromise the entire operating system and be controlled remotely by an attacker.  Anti-malware software exists specifically to combat the threat of malware programs, assisting with detection/removal and can potentially prevent infection in the first place.