I think most of the websites while registering (Sign up) our account in their sites they asks us to enter some code as it shows a distorted image like above. For example in Yahoo, G-Mail while we are registering a new e-mail account it asks. Some websites asks like this "Are you a human?". Then we have to type a code in box below it. Till now I observed lot of websites and just left it. While registering my friend's account (Who is newbie to internet) in yahoo he aked "Why to type that code?". Then I started to think "Why?". Now I got that answer by searching on google.
That is here for you.
"Now a days computers and Internet have evolved to the point when software tools ( such as standalone programs or web scripts ) can perform automated task on the web, such as navigating from website to website ( spiders ) filling and submitting web forms.
This task was difficult many years ago, and people did not thought much about it back then. But now, with the increasing amount of SPAM and the advanced script languages, many hackers and spammers are writing and using scripts to automatically fill and submit forms."
The mistory behind of the code requires that the user have to type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. This usually ensures that only humans can enter the correct answer, because is difficult ( but not impossible! ) for machines to extract the letters and digits from the image. So finally what I concluded my self is that, to prevent spam websites uses that code.
Here is site to accomplish above task in your website http://www.captcha.net/. This provides free service to prevent spam in your websites. For more information go through it's home page.-