Put simply, a keylogger is a program that will intercept every single keystroke that is made on your computer and store it. The kind of software that spies and hackers love to use as it exposes your innermost secrets - passwords, chats with "close friends" on the internet, that kind of thing. Depending on which side of the program you're on - the user being spied on or the person doing the spying - it's either a good or a bad thing. So let's take a look in more detail.

It's another computer program that runs on your PC. A bit like an instant messenger program or Skype, it will probably be set to run for every user on your machine, every time Windows starts.

Because the program doesn't really want people to know it's being used, it will probably run in "stealth" mode by default. So it won't announce itself as starting when Windows starts up. And it probably won't be shown in the list of programs that are shown as running by Windows Task Manager. Plus it will have an obscure sounding name in the list of processes that are running - it won't be anything obvious like IamWatchingYou.exe otherwise the person you're keystroke logging would know.

The program will then sit quietly in the background, much like lots of other background tasks that happen all the time in Windows.

Some hardware keylogger programs will offer you the option of taking snapshots of the computer screen as well as recording everything that happens on the keyboard. If you're trying to confront your kids or your partner with whatever misdeeds they did, this can be a useful weapon.

You can then log on at a later date and see exactly what has happened on the computer since the key logging started. For the nerdier people among us, these programs also show every time the delete or backspace key was hit - which isn't just useful for seeing how good or bad someone is at typing, it's also quite fun to see how they changed their mind between almost sending something as a message and then not doing so.

A keylogger program has two areas. The regular Windows install, which is simply a matter of following the seemingly never ending quiz that near enough all programs go through between pressing the Install button and actually getting the darn thing to run on your computer. That's normally just a matter of clicking Yes or OK umpteen times.

This usually involves pressing a combination of 3 or 4 keys all at the same time. These will be at awkward places on the keyboard, which is actually good news because you don't want the person you're spying on to accidentally trigger the "I know what you're doing" screen.

If you need to keep an eye on your children, it's a great way to check that they're not being inadvertently groomed and that the videos they tell you they're not looking at aren't corrupting them.

It's also useful for monitoring what your employees are getting up to and making sure that they're working more often than they're checking Facebook.