Most sources define a keylogger as a software program designed to secretly monitor and log all keystrokes. This definition is not altogether correct, since a keylogger doesn't have to be software--it can also be a device.

Unlike other types of malicious program, keyloggers present no threat to the system itself. Nevertheless, they can pose a serious threat to users, as they can be used to intercept passwords and other confidential information entered via the keyboard. Check out Kaskpersky's take on keyloggers in <A HREF="http://www.viruslist.com/en/analysis?pubid=204791931">this all-about article.</A>