Have you seen a squiggly/corrupted image with some numbers/letters in different directions when you are registering or filling up a form in any web site? Just to make you understand what I am referring to, try to create a new Yahoo mail account. In the registration form there is a section titled “Verify your registration” and you can see the image I was talking about. That is called CAPTCHA. The CAPTCHA image could contain randomly generated letters and numbers in a distorted fashion only human mind could interpret. Why is it needed? Well there are robots and spiders (applications that spy the net) that try to misuse the web applications. These applications can mimic the input forms and submit it to a web application but it is way too hard to interpret the distorted image. This is one important security measure that can help avoiding bots spamming your application. I have used CAPTCHA in my blog. Where? Click on the comments link and you will see it.
Creating a CAPTCHA requires generating images at run time. There are open source technologies like GD and specifications like SVG (Scalable Vector Graphics) that can help create an image at run time. The CAPTCHA page on Wikipidea has programming language based implementation references.