What is Key Logging?
Keylogging is the capturing of everything entered using the keyboard and storing this information. Viruses, trojans and worms can contain keyloggers and thus log everything you type, and send it to the creator of the malware. Not a real problem when you only use our computer for editing and printing word documents.
Think again, a keylogger wil save EVERYTHING you enter using the keyboard, including passwords, the, sensitive, information in the document and much more. Much more, well, everything, imagine having your creditcard info out in the open. Luckily most modern virusscanners can detect keyloggers even unknown keyloggers, since most of them use certain code which most keyloggers contain. Nevertheless the risk, how minimal it may be, is still a risk.
You get a keylogger the same way you get other malkware, bad protection or unsafe surfing, to remove one, go here. And to prevent problems in the future go here.