This is a Captcha
Here is the Webopedia definition:
Short for completely automated public turing test to tell computers and humans apart, a technique used by a computer to tell if it is interacting with a human or another computer.
via What is CAPTCHA? – A Word Definition From the Webopedia Computer Dictionary.
According to the experts machines are getting better at reading captchas so we can expect the to become even more obscure.