A check used to tell real people from bots.
CAPTCHA is the website bouncer with a pop quiz. It asks for traffic lights or twisty letters before you enter.
You meet it when you log in or sign up, and ticket sites use it too. It blocks bots, but it can trip up real people.
Turing-test
CAPTCHA flips the Turing-test and makes you prove you are human.
Agent
Many sites use CAPTCHA to stop an Agent from signing up or buying tickets at scale.
Computer use
When AI uses a web page for you, CAPTCHA is often the first roadblock.