Captcha comes from the so-called Captcha test, which is used to tell a real person from a programmed machine or an automated program crawling unsecured websites.
A Captcha test is used in many kinds of forms, such as registrations and replies to classified ads, in the form of a distorted or otherwise altered image.
Use
The internet is full of programs and “spamming” bots that crawl websites one after another, looking for forms or discussion forums that are not secured in any way. If they find an unsecured website, they start automatically posting free ads and backlinks into the conversations.
Another use is securing sites such as file-sharing services, where a Captcha test protects data from being scraped, that is, automatically downloaded by automated programs.
One interesting fact is the reCAPTCHA project, which, instead of randomly generated texts, picks real words from old digitized books.