Malware (a portmanteau for malicious software) is any software intentionally designed to cause damage to a computer, server, client, or computer network.

A wide variety of types of malware exist, including computer viruses, worms, Trojan horses, ransomware, spyware, adware, and scareware.

Programs are also considered malware if they secretly act against the interests of the computer user. For example, at one point Sony music Compact discs silently installed a rootkit on purchasers' computers with the intention of preventing illicit copying, but which also reported on users' listening habits, and unintentionally created extra security vulnerabilities.

A range of antivirus software, firewalls and other strategies are used to help protect against the introduction of malware, to help detect it if it is already present, and to recover from malware-associated malicious activity and attacks.

The best-known types of malware, viruses and worms, are known for the manner in which they spread, rather than any specific types of behavior.

More Info: en.wikipedia.org