There's the common "distorted text" CAPTCHA which requires the user to enter text displayed in an image designed to be difficult for a computer to read, but those are getting increasingly easier to beat with software. The idea is that some functions (such as posting a comment on a forum) should be done by real humans only and not automated processes. It doesn't really "secure" a website, it just makes it increasingly difficult for an automated system to access that functionality of the site. Its purpose is as a challenge-response test to demonstrate that the person using it is a human being and not an automated program. This process has no affect on the user of the website as it is only the first word that is used to determine whether they are a robot. So the word is made available in the CAPTCHA of a participating website and the meaning is determined. The Optical Character Recognition (OCR) is not perfect and sometimes the meaning cannot be determined.
This is performed as a public service to organisations such as Libraries and Public Archives that are scanning large numbers of historical documents. See this site (thanks The first word is used for validating the user and the answers to the second word are compared to other users' answers for that word and in this way the probable meaning of the text is determined. The CAPCHA with two words usually consists of a word known to the Web Application and a second word that it is trying to decipher. One has a single combination of characters that the user has to enter, the other, such as on SO has two. Here on Stack Overflow, if you edit answers or questions a number of times, you will be prompted before further edits are accepted. It is used for logins as well as on other data entry forms. The text can also be in the form of a sound, as per comments See this site. It prevents hackers from posting forms using automatic scripts, by requiring the user to input data read from images which are difficult to read automatically. Completely Automated Public Turing Test To Tell Computers and Humans Apart
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