CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart.
It is a security mechanism used on websites and online services to distinguish between human users and automated bots or scripts.
The main purpose of CAPTCHA is to prevent bots from carrying out malicious activities, such as spamming, brute force attacks, data scraping, and other automated actions that can compromise the integrity and security of a website or service.
Basically, CAPTCHA is an image with text shown on it. The user must type the same characters as the ones on the image to verify s/he is human.
It is often used in forums and contact us pages to prevent entries from spambots.