Google reCAPTCHA is a free CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart) service designed to identify humans from bots and to protect websites from spam, phishing, and hacker attacks. It uses an advanced risk analysis engine and adaptive challenges to keep automated software from connecting with abusive intents to a website. Since these challenges are only solvable by humans, it allows only the welcomed visitors to gain access to the website.
Google reCAPTCHA can be integrated into any website or web application with API key pair. It consists of a Site Key, intended for invoking reCAPTCHA service on the site or app, and a Secret Key intended for authorizing the communication between the backend and the reCAPTCHA server.