With the help of Captcha, which is commonly unreadable for the robots, the spam is blocked so it can`t get into the database. The Captcha numbers image has a fixed background and a random number combination, that the user has to enter in the intended field.  

On some forms you have to enter the numbers you see in the specified field:

For more information see:

1. http://www.captcha.net/ 

2. http://en.wikipedia.org/wiki/Captcha