Domain Social Forums

Could anyone explain how captchas work?
Is there any particular format of captcha or we can use any type of captcha to our website?

On the surface, captchas look easy to use and interact with. Well, they only look that way to people who have perfect vision and an elastic mind. If you have these qualities, congratulations.

Worst of all, even people with super sight and brilliant spatial awareness find captchas plain annoying. This drives away users, which drives down revenue.