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Captcha comes from "Completely Automated Public Turing test to tell Computers and Humans Apart". It is used by web sites to distinguish human web users from robot spammers.

Recently, in just minutes, an artificially intelligent machine cracked ond of those jumbled text sequences. The smart machine can be trained in a matter of minutes using just a few hundred example characters, researchers said. It works with multiple different styles of captcha and can also be re-purposed to identify handwritten digits, recognize text in photos of real-world scenarios, and detect non-text objects in images. That is because the company designed the system to mimic the way the brain identifies objects after seeing just a few examples and recognize them in strange new configurations.

Text-based captchas as above work, because unlike humans, computers struggle to recognize the distorted and partially hidden characters that make them up. Though many current machine-learning systems that can solve them exist, they must be trained on millions of images to work.

Many websites have moved away from text-based captchas, and are using image-based tests and data on mouse movements, or cookies to analyze whether users are human or machine. Some captchas are also difficult for humans to accurately decipher on first try.