 Captcha
CAPTCHA is a Completely Automated Public Turing test to tell Computers and Humans Apart. Most forums or website user can post on use this system so no automated scripts can add posts with links in them. This is useful because links from other sites boost PageRank and therefore many people try to automate the placing of links on other websites. A CAPTCHA is usually a image with letters in it that need to be inputted in a formfield. | 
