You're not alone if you haven't heard that CAPTCHA acronym before - neither had I.
However I have come across them many times - Network Solutions requires you to pass a CAPTCHA test before every "whois" query, and many other sites (particularly 'free' services) are not asking new users to 'verify' themselves via CAPTCHA...
So what is it? A combination image & form input which requires you to 'decipher' some distorted/disguised/obsfucated text from the image and type it into the browser. It's a LOT easier for people to read these images than it is for automated bots to OCR or otherwise guess them - thus preventing automated/fraudulent use of your website/resources!
Interesting, eh? This article 15 Seconds : Fighting Spambots with .NET and AI is a great intro, and includes some interesting examples and Visual Basic.NET sample code. See the CAPTCHA references too.