It's computer monitoring software, the basic ones are keyloggers. There are free ones, which basically only record keystrokes, to the more expensive ones that record keystrokes, capture screenshots of the computer, email you the results, etc. There are good ones like Spector Pro, Web Watcher, Spy Agent, etc. There are also free ones, but you get what you pay for. Monitoring Software Review 2011 - TopTenREVIEWS
Why do you need a keylogger? So you can be aware of their communications, such as how long this has been going on, what they have done, and what they are planning to do. Another key tool cheaters use is the secret email account. A keylogger will capture their usernames and passwords.