A "Captcha" is a challenge-response method for verifying that a person is actually filling out a form, and not an automatic form filling spam bot from hell. It usually consists of letters and numbers on a background to scramble the pattern so it cannot be read by a machine. Sometimes it's a question, sometimes it's a math equation.
Here are some funny and unfortunate Captchas courtesy Docstoc.com.
Link: http://www.docstoc.com/docs/1048763/Worst-Captchas-of-All-Time