I am not a robot - this simple sentence to tick off, for example when registering for an account in an online shop, guarantees that a human and not a machine or malware is using the website. This is how reCAPTCHA, a tool to protect a domain from spam, works.
reCAPTCHA - what is it and how does it work?
reCAPTCHA is a free tool offered by Google that protects websites from spam bots and blocks them from entering any data into the website. This method makes it possible to verify that a registration or contact form is actually being used by a human being and not by spam robots that automatically register accounts on portals or post comments en masse on forums and blogs.
The main objective is to protect and secure forms and prevent servers from slowing down. The tool is also intended to protect site owners from automatically generated spam posts and thus prevent the distribution of harmful content on the site. 
CAPTCHA stands for: Completely Automated Public Turing test to tell Computers and Humans Apart, which loosely translated means: a completely automated public Turing test to distinguish computers from humans.
The initial form was a text-based version, where the user had, for example, to assemble a word from a distorted combination of letters and numbers. Other methods included, for example, selecting on a random image those elements containing, for example, a bicycle. However, this did not provide full protection against spam, and could annoy users when they had to run an additional command once again because they had inadvertently solved a puzzle inaccurately. Therefore, the reCAPTCHA method was introduced.
This is an easier solution for users, while also giving a better effect of protecting the site from spam. The service was developed at Carnegie Mellon University in Pittsburgh and later acquired by Google.
The service runs in the background and, based on data collected by Google, is able to detect suspicious user behaviour. At such times, it is necessary to confirm that you are a human, usually by clicking a box next to "I am not a robot" (a so-called checkbox). After verification, the user can continue and, for example, post a comment or create an account. 
reCAPTCHA - the most important types
The most common method of verification with reCAPTCHA is to tick the box that says: I am not a robot. The user makes just one move and tasks, and the software knows that a human is sitting on the other side.
Another popular form is a message with a picture divided into boxes. The user has to tick those puzzles where there is a particular thing or object, such as a car or a traffic light.
How to launch and implement reCAPTCHA on a website?
To run and implement reCAPTCHA on your website, you must have a Google account. After logging in, go to the reCAPTCHA subpage where you need to add a site, fill in the form and submit the data. Once you have received the keys, you will need to enter them into the site code. You can also use plug-ins for example for WordPress.
Summary
A website that is well protected against spam is more user-friendly, not least because harmful content is not distributed. In turn, site owners can not only better monitor what is happening on their site, but also have greater confidence in the data they are acquiring, ensuring that, for example, genuine addresses are included in the mailing database.
To summarise in one sentence, reCAPTCHA is a security service that protects websites from fraud and abuse.
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