via wpwhitesecurity.com => original post link
Since CAPTCHA was first introduced, it has undergone various iterations and evolutions. With each step, the aim always has been to make it easier for humans and more challenging for non-humans to pass the test. Over time, this led to several different types of CAPTCHA checks being used.
CAPTCHA tests must also consider accessibility features such as screen readers used by visually impaired people. Since these function like a bot, it can make the entire process somewhat counterintuitive. Even so, this has been one of the motivators behind the evolution of CAPTCHA checks.
It is also important to note that CAPTCHA tests are offered by different vendors. Websites that offer such tests connect to the service provider who runs the actual tests. As such, you might not be able to find all different types of checks from any one vendor.