CAPTCHA

Wikipedia's ReCAPTCHA test, used before making certain changes or creating accounts. The letters are completely random.

A CAPTCHA is a test that is used to separate humans and machines. CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." It is normally an image test or a simple mathematics problem which a human can read or solve, but a computer cannot. It is made to stop computer hackers from using a program to automatically set up hundreds of accounts, such as email accounts. It is named after mathematician Alan Turing's Turing test.^[1]

CAPTCHA Media

This CAPTCHA (GIMPY-R, c. 2005) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.
Many websites require typing a CAPTCHA when creating an account to prevent spam. This image contains a user trying to type the CAPTCHA word "sepalbeam" to protect against automated spam.
An example of a reCAPTCHA challenge from 2007, containing the words "following finding". The waviness and horizontal stroke were added to increase the difficulty of breaking the CAPTCHA with a computer program.
A CAPTCHA usually has a text box directly underneath where the user should fill out the text that they see—in this case "sclt ..was here".