CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". It is a type of security measure known as challenge-response authentication that helps protect against spam and unauthorized account entry by asking users to complete a simple test that proves they are human and not a computer trying to break into a password-protected account. A CAPTCHA test is made up of two simple parts: a randomly generated sequence of letters and/or numbers that appear as a distorted image, and a text box. To pass the test and prove your human identity, simply type the characters you see in the image into the text box. CAPTCHAs are used by many web services, including Google, to help prevent unauthorized account entry. They are also used on other sites that provide access to sensitive information, such as bank or credit card accounts. CAPTCHAs can be added to online forms, and there are different types of CAPTCHAs available, including image-based, audio-based, and math-based tests.