Is There a Difference Between Humans and Computers?

Yahoo wanted to stop people from writing programs to obtain millions of email accounts for free. So they came up with a test that can distinguish whether the thing getting an email account is actually a human or a computer. At the time, computers were not very good at reading images of distorted text, but humans were. A computer starts by putting some letters on an image, then it distorts them. All you have to do is just match the sort of letter.