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:
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http://www.captcha.net/
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http://en.wikipedia.org/wiki/Captcha