[ See also: Free antivirus software: The best and the rest ]
Since they pass or store so much valuable and sensitive data, such devices are now a tempting target for criminals, who try to gain uninvited access. This is usually done by first installing - or getting the user to install - dangerous software on the PC, which is usually invisible to casual inspection.
As well as sending sensitive data stored on them to unknown attackers, PCs are sometimes hacked in order to take control for other unlawful hacking use. For example, most of the world's spam email is sent via compromised Windows PCs, all unknown to the PC user. Many PCs colectively under the control of a cyber-criminal organisation is known as a botnet.
Most attempts to break into computers are aimed at Windows PCs, in part because Microsoft still has a near worldwide monopoly on personal computers, making them far more abundant; and in part because Windows is an inherently less-secure platform, designed before the internet age, and easier to compromise.
Malware is the name given to any combination of virus, Trojan, worm, spyware, rootkit or keylogger, usually designed to allow a remote attacker to gain control of your computer.
Security software is designed to prevent your PC from becoming infected with malware in the first instance. Additinally, it should also help remove malware that it let slip through and is now already installed.
Note that security software will not protect you from security holes and vulnerabilities that may exist in the computer operating system. Nor will it prevent your computer being taken over by security flaws in third-party software on your PC, such as Adobe Flash and Adobe Reader.
The simplest security software is sold as 'anti-virus'; in fact, it's usually designed to indentify and block any of the usual malware variants of virus, Trojan, worm, etc.
Anti-virus software today usually takes a two-pronged approach to spotting malware before it reaches your computer: signature identification, and heuristic detection.