CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. Developed and trademarked by Carnegie Mellon University, the job of this test it to know if it's a person or a computer trying to access your system or website. Read more about CAPTCHA at http://www.protectwebform.com/.
When you take the test that CAPTCHA gives you and give the right answer, it knows that you are a human. For instance, it will ask you to key in letters from a distorted image of alphabets and numbers. The machine will not be able to figure it out correctly. That makes it quite easy for CAPTCHA to know the difference. You can get further details from http://www.protectwebform.com/smartcaptcha.
Website owners have to guard against spammers yet they need feedback from the visitors to their site. Spammers are always on the look out to grab your email id and use it for various purposes without your permission. One option is that you can go in for automated programs, called bots, to let them scan your website and parse out your mail id.
Another option is CAPTCHA that does not allow the spammers to get access to your email ids. Many websites and sign-up pages use CAPTHCA. It assumes that computer programs cannot read distorted characters. It is considered fairly reliable.
As a fully automated program, CAPTCHA is so advanced it creates tests that can only be completed properly by humans. And since it runs itself, you don't have to baby-sit the system for it to work. This is why so many webmasters use CAPTCHA to keep their sites safe and secure.
CAPTCHAs prevent abuse of a system. Once spammers get access to your mail id, then you are trapped. You will be bombarded with spam mails and you can never be sure what your id will be used for.
Many email services and online blogs are the ones who could benefit most from CAPTCHA. If you have webmail accounts through Gmail, Hotmail, or Yahoo, you know how many spam e-mails seem to get into the system. In addition if you regularly read online blogs, you have likely seen some that have had a long string of spam postings. CAPTCHA can stop these posts from happening by catching the spamming programs before they have a chance to do their work.
There have been numerous efforts at creating CAPTCHAs that are handier. Such endeavours usually include the use of JavaScript, mathematical questions (like "what is 1+1"), or simple questions requiring common sense (for instance "what color is the sky"). These attempts infringe on one or both of the philosophy of CAPTCHAs. Either they cannot be automatically created or they can be easily split given the condition of false intelligence. So, the only security these CAPTCHAs provide is safety through obscurity.