Captcha. No, it's not a game of tag!

If you've recently completed an online form, you may have seen at the end of it, a distorted image containing numbers and/or letters which you need to re-type before you hit 'send' or 'sign up'. This is needed to prevent 'bots' (net nasties) from filling in these forms themselves. By filling this in correctly, the computer knows it's a nice human instead

Here's an example of what they look like: