OAuth2
In plain terms
The industry-standard protocol for delegated authorization.
Going deeper
OAuth 2.0 lets apps obtain limited, revocable access to user resources via tokens without sharing passwords, through defined grant flows. It powers 'sign in with' and API access.
Under the hood
Grant types (authorization code, client credentials, PKCE) suit different clients; it's authorization, not authentication.
Examples
'Login with Google', API access tokens, third-party authorization, PKCE.
Think of it like...
Like a valet key that lets a service drive your car but not open the trunk.