Una breve explicación en I'm Not A Robot:
CAPTCHA stands for Completely Automated Public Turing Test to tell Computers and Humans apart and it’s a way to filter out bots and fraudulent automated activity from the behaviour of real people. It's an umbrella term describing several different techniques presented to the user to determine if they are human. A CAPTCHA challenge could be a random collection of letters and numbers, text obscured with background noise, puzzle challenges or audio challenges asking the user to enter the letters and numbers heard with a lot of background static noise. All of these are termed CAPTCHA as they're asking the user to demonstrate they're human and not an automated computer program.
The theory is humans are very good at being able to identify distorted text, numbers and audio but not a computer program. A computer program i.e. bot can't reliably identify displayed text or audio and so it's a very effective way to stop bot activity affecting your website.
The problem is CAPTCHA in its many incarnations causes significant challenges for people with disabilities. Asking a user to decipher distorted text may mean vision-impaired people will be unable to complete it. Presenting an audio challenge may mean people with a hearing impairment will have difficulty, reorientating a visual 3D puzzle may affect users with mobility and cognitive impairments and disabilities are rarely isolated, users may have a range of disabilities.
If your security check is relying on some kind of user input to determine the "humanness" of the person at the other end, it is ultimately doomed to failure.
