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
numbers]
Re-type it here [you have to enter exactly what is show above]

Any help will be appreciated.

It's called Captcha

http://en.wikipedia.org/wiki/Captcha

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

http://woork.blogspot.com/2009/02/10-free-captcha-scripts-and-services.html

Thanks Lee, this helps me a lot.