Google reCAPTCHA is a bot protection service for websites and mobile apps, now part of Google Cloud Fraud Defense. A bot here is a program that uses a site automatically, for example to send spam through a form, try stolen passwords (credential stuffing) or create fake accounts.
The site adds Google's script or SDK. reCAPTCHA analyzes user behavior and other signals and returns an assessment: a risk score that says how likely it is that an action comes from a bot. Version 3 works without user interaction; instead of showing a CAPTCHA challenge (a small test that people pass and bots usually fail), it returns the score, and the site decides whether to allow, check or block the action; reCAPTCHA can also show a challenge or block the action itself. The paid tiers add password defense, account takeover detection, and protection against SMS toll fraud and fraudulent payments; reCAPTCHA can also feed a web application firewall.
reCAPTCHA is a hosted Google Cloud service. The Essentials tier is free up to a monthly number of assessments; the Premium tier is billed by monthly volume, and Enterprise is a yearly subscription.