Its purpose is a challenge-answer test to demonstrate that the person using it is a person, not an automated program. In fact, it does not “protect” the website, but simply automates the system to access this website functionality. The idea is that some functions (for example, posting comments on a forum) should only be performed by real people, not automatic processes.
. " " CAPTCHA, , , , , . CAPTCHA, , , . , , , , . - , " " - , - , .
. : http://en.wikipedia.org/wiki/CAPTCHA
. Captcha.net: http://www.captcha.net/