This document discusses CAPTCHAs, which are programs that generate tests to distinguish humans from bots attempting to access websites. It defines CAPTCHAs and provides a brief history, describing the first CAPTCHA developed by AltaVista in 1997. The document outlines different types of CAPTCHAs, including text, graphics, and audio-based, and discusses some applications and how CAPTCHAs can be broken, along with benefits and drawbacks.