It's called a Captcha and, according to Wikipedia (the "Source"), a Captcha is a "type of challenge-response test used in computing to ensure that the response is not generated by a computer. The process usually involves one computer asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human."
Here's a Captcha I came across today:
And then they superimpose a web over it:
and you end up with something like this:
which is very, very annoying, particularly on a full stomach.