Short for "Malicious Software."
Malware is a category of software programs designed to damage or do other unwanted actions to a computer system. Hackers generally develop malware for some criminal purpose — like stealing protected information or taking control of a computer to add it to a botnet — although some create malware solely to spread damage. Anti-virus and anti-malware software can monitor a computer to prevent malware from infecting it.
There are several common types of malware, categorized by how they infect a computer and what they do once in place. For example, worms and trojan horses are types of viruses defined by how they spread; worms spread automatically, while trojan horses trick a computer user into installing malware. Spyware, ransomware, and rootkits are instead defined by what they do. Spyware spies on you to steal your personal information like banking info or passwords, ransomware encrypts your files to hold them for ransom, and rootkits provide a hacker with root-level access to your computer.
Anti-virus and anti-malware utilities can help protect your computer against malware, but only if you install and run them. Current versions of Windows include a built-in anti-virus utility, while other operating systems support third-party anti-virus software instead. However, be careful and only install anti-virus software from a trusted source; fake anti-virus software that actually includes malware is increasingly common.