what is reCAPTCHA in websites ? and how it work

• this time a post about all people to surfing lost of  website and you can see reCAPTCHA checkbox.
• so let's start...

reCAPTCHA website's

• this recaptcha is product by google and there to free service to used for your website.
• Question is used or not used this recaptcha ?   
• your site commercial lick online selling ,online BANK's and other earning site's you used reCAPTCHA and secure you site ..   
• secure ! yes secure for spam and other cyber attacks or hackers.

So Question is How it work..??..

•  reCAPTCHA is working to create some pictures for unlock the next URL. lick see this images. 

•  You can select images for as question .
• after click verify.