For example, some comments boards require a CAPTCHA to be entered to prevent bots from repeatedly submitting advertising content.
They may also be used when users log in to certain accounts, in case a bot is systematically submitting vast quantities of credentials to try and find successful combinations. These could then be used to gain access not only to that account but to other accounts where people have used, for example, the same email address and password.
CAPTCHA stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart.
Completely Automated Public Turing test
Standard Definition Exists?
A computer program or system intended to distinguish human from machine input, typically as a way of thwarting spam and automated extraction of data from websites.
Year first used/coined:
Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford