How to check for a real person?

Some sites have some checks to make sure that the data is being sent
by a real person.
Something like this:

See this code? [there is a image here with random letters and
Re-type it here [you have to enter exactly what is show above]

Any help will be appreciated.

It's called Captcha

Here's 10 examples (reCaptcha probably being the most common):

Thanks Lee, this helps me a lot.