Web applications often use CAPTCHAs to ensure that real human beings and not bots are using the system. These CAPTCHAs are mostly a combination of words that are distorted enough so that only human eyes can read and solve them.
Facebook is however taking a more social approach to CATCHA solving.
Traditional CAPTCHAs are often hard to solve and therefore Facebook is experimenting with social authentication where you don’t have to recognize letters but images – they’ll show you a few pictures of your friends and ask you to name the person in those photos.
Also see: Cats as CAPTCHA