A malicious program that records keystrokes. Often, a keylogger also keeps track of visited web sites, and time-stamps when you visit, as well as what keys you type. More advanced keyloggers can limit their recordings to certain prompts / fields (like the user-name and password boxes) and for only 'secure' web sites (like you bank or EBay etc.) Since the keylogger records information as you type it, as opposed to intercepting it when you submit information on a web page, the information is unencrypted even from secure web sites. Keyloggers are one of the most dangerous forms of malware (due to identity theft), and therefore it is recommended that if you find one, immediately unplug your ethernet / modem cable, back up your important files, and reformat.