An anti-spam technique that was developed to block robots from spamming a site. CAPTCHA works by asking the user to answer a question or solve a problem that computers were supposedly incapable of solving but humans were.
Types of CAPTCHA include:
- Asking the user to read a bit of hard to read text in an image.
- Asking the user to solve a math problem (such as "what is 2 + 2").
- Asking the user to answer a question (such as "what is the name of this site?").
Robots have gotten better at reading text in images over the years, which is why some people advocate using option 2 or 3 instead.