Implementation
The reCAPTCHA tests are displayed from the central site of the reCAPTCHA project, which supplies the words to be deciphered. This is done through a JavaScript API with the server making a callback to reCAPTCHA after the request has been submitted. The reCAPTCHA project provides libraries for various programming languages and applications to make this process easier. reCAPTCHA is a free service (that is, the CAPTCHA images are provided to websites free of charge, in return for assistance with the decipherment), but the reCAPTCHA software itself is not open source.
Also, reCAPTCHA offers plugins for several web-application platforms, like ASP.NET, Ruby, or PHP, to ease the implementation of the service.
Read more about this topic: Re CAPTCHA