OpenID is an open standard and decentralized authentication protocol. It allows users to be authenticated by cooperating sites (known as relying parties, or RP) using a third-party service, eliminating the need for webmasters to provide their own ad hoc login systems, and allowing users to log into multiple unrelated websites without having to have a separate identity and password for each.
It is identity layer on top of the OAuth 2.0 protocol. OpenID-Connect is based on most modern protols. It uses JSON tokens, signed and optionally encripted using JWT standard, and uses simple REST as its transport protocol.
Sometimes referred as OpenID, must not be confused with an older and deprecated standard named OpenID.