A CAPTCHA, or Completely Automated Public Turing-test to tell Computers and Humans Apart, is used to keep bots and other automated programs from signing up for offers, collecting or signing up for email addresses, violating privacy, trying to crack passwords, or sending out spam to unsuspecting email recipients. A CAPTCHA works by issuing a challenge to the person or entity attempting to gain access.

A CAPTCHA challenge is usually a simple visual test or puzzle that a sighted human can complete without much difficulty, but that an automated program cannot understand. The test usually consists of letters, numbers or other images that overlap or intersect. The images are distorted in some way or shown against an intricate background to keep them from being easily read by another computer.