What is CAPTCHA and what are the differences between CAPTCHA, ReCAPTCHA, and NoCAPTCHA?

CAPTCHA is the general term used to describe an online validation process that identifies whether the connection made on the website is from a person and not a bot.
The name is not a word in itself, but an acronym for the English terms "Completely Automated Public Turing test to tell Computers and Humans Apart" and as the name suggests, it is a completely automated process, so there is no need for anyone to check whether the test has been cheated or not.
Websites that use CAPTCHA do this through a provider. This provider offers a kind of API to which the website connects. The provider then runs the actual test and sends the result back to the website. There are several providers offering CAPTCHA services today, including Google.

reCAPTCHA

reCAPTCHA is the name of a CAPTCHA service provided by Google, currently available in two versions: reCAPTCHA v2 and reCAPTCHA v3.
Version v2 was officially launched in 2014 and has two forms: “I’m not a robot” and "Invisible".

The new variant v3, which is still in use, employs a score-based algorithm where a user is assigned a score between 0 and 1 in decimal format. The closer the score is to 0, the more likely it is that a user is a bot.

noCAPTCHA

This is a different type of CAPTCHA compared to traditional ones, in that there are no texts to fill in or images to confirm, as these are hidden from the user.
Using a complex algorithm, it analyzes on-page behavior to determine whether it resembles that of a human or not. Its purpose is to streamline CAPTCHA tests so that they are not intrusive for users and are more secure for website administrators.
However, there are situations where the noCAPTCHA algorithm cannot accurately predict the displayed behavior, so in some cases a traditional CAPTCHA test is presented to the user.