As we developed websites and apps, others developed automated programs, or bots, to infiltrate them, sometimes to hack, but mostly to spread spam or skew statistics. So developers came up with Turing tests called CAPTCHAS to eliminate non-human users. But the bots got better, and so the tests had to get better, in an escalation that never ends. Tom Scott explains the process. This video is barely over five minutes- the rest is an ad.
Source: neatorama