Captcha stands for Completely Automated Public Test to tell Computers and Humans Apart.

Captcha is a type of challenge-response test used in computing to determine whether or
not the user is human.

This can be achieved by displaying an image to the user contain some letters and the
user should enter it. Some programs can make pattern recognition and parse the
images and enter it again, so it is a challenge to create a readable, complex
and confusing image.

http://www.egjug.org/using_jcaptcha_in_your_j2ee_web_forms

Regards,
Ahmed Hashim