CAPTCHAs are manipulated words that you have to type in correctly to prove that you are a human. They are designed to be difficult to read so a computer cannot recognize the letters. The idea is to prove the person using a site or typing in a password is in fact a person.
D&B, a site offering credit reports, also has CAPTCHAs.
Except that D&B's CAPTCHA is just a fake. The secret text is included in the HTML code. So a computer can definitely read it.
In some ways, D&B has created a way to tell computers and humans apart. Computers can read it and humans cannot.
http://svedic.org/programming/worst-captcha-ever was the first person to identify the flaw.