CAPTCHA stands for "Computer Automated Public Turing test to tell Computers and Humans Apart." Essentially they are those squiggly letter things that many of you have had to fill out from time to time. A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text, but current computer programs can't.
How CAPTCHA Works:
They stop automated programs from trying to send you spam through your contact form because only humans can read and type in the letters.
Applications:
Preventing Comment Spam in Blogs: Most bloggers are familiar with programs that submit bogus comments, usually for the purpose of raising search engine ranks of some website (e.g., "buy penny stocks here"). This is called comment spam. By using a CAPTCHA, only humans can enter comments on a blog. There is no need to make users sign up before they enter a comment, and no legitimate comments are ever lost!
Protecting Website Registration: Several companies offer free email services. Up until a few years ago, "bots" would sign up for thousands of these email accounts every minute. The solution was to use CAPTCHAs to ensure that only humans obtain free accounts. In general, free services should be protected with a CAPTCHA in order to prevent abuse by automated programs.
Online Polls:
Can the result of any online poll be trusted? Not unless the poll ensures that only humans can vote.
Preventing Dictionary Attacks:
CAPTCHAs can also be used to prevent dictionary attacks in password systems. The idea is simple: prevent a computer from being able to iterate through the entire space of passwords by requiring it to solve a CAPTCHA after a certain number of unsuccessful logins.
Search Engine Bots:
It is sometimes desirable to keep webpages unindexed to prevent others from finding them easily. However, in order to truly guarantee that bots won't enter a web site, CAPTCHAs are needed.
Worms and Spam:
CAPTCHAs also offer a plausible solution against email worms and spam: "I will only accept an email if I know there is a human behind the other computer." A few companies are already marketing this idea.
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