Malware comes in many forms … and many disguises. Variants include computer viruses, worms, Trojan horses, rootkits and ransomware, but it all comes down to the same thing: malicious software invading your machine.
The main purpose of malware is to steal money, data, access and other valuable things. It may be either self-propagating or controlled by a remote attacker, but any form of malware attack on your system will be dangerous for you.
To protect yourself from malware, always use an updated antivirus and firewall. Enable automatic updates so that your system doesn’t miss crucial patches. But you have an important role in protecting your system, too: it’s important to never carelessly download files or permit random programs to run. In the chaos of the Wild Wild Net, that could be malware searching for a way in!