CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart" and means software peace which makes user input challenges which are easy for solving by humans and difficult for machines. The purpose is to avoid spam or hackers attacks which can harm your data or make unpredictable responses.
This post will show you one very good CAPTCHA control, which is tested by me and works fine, even with ajax. It is easy for use and will save you time to develop such control.
Thanks for Mondor software.
http://www.mondor.org/captcha.aspx
