CAPTCHA
A small test a website uses to tell a person from an automated program.
A CAPTCHA is the little challenge that appears before you sign up, log in or buy tickets: type the wavy letters, click every picture with a traffic light, or just tick a box that says "I'm not a robot". The name is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". The idea is a task that is easy for people and hard for software.
It exists because bots are cheap. A single program can try thousands of stolen passwords per minute, create fake accounts by the hundred, post spam or buy a whole concert in seconds to resell it. A CAPTCHA does not stop a determined attacker, but it makes each attempt slower and more expensive, which is often enough.
The tests have changed a lot. Distorted text became easy for image-recognition software, so many sites moved to "invisible" checks such as Google's reCAPTCHA or Cloudflare's Turnstile: they look at signals like how the mouse moves, the browser and the network's reputation, and only show a puzzle when something looks off. The trade-offs are real: puzzles can be a wall for people with visual or motor disabilities, and the invisible kind works by observing your behavior.
Keep in mind
- Its job is telling people apart from bots, not checking who you are.
- It protects sign-ups, logins, forms and ticket sales from automated abuse.
- Modern versions are often invisible and only show a puzzle when something looks suspicious.
- Puzzles can block people with disabilities, so good sites offer accessible alternatives.
