What is the difference between OAuth 2.0 and OpenID Connect?

OAuth 2.0 is an authorization framework: it issues access tokens so a client can call APIs on a user's behalf — it says nothing standard about who the user is. OIDC is an identity layer on top of OAuth that adds an ID token (a signed JWT) and a userinfo endpoint for authentication. Using a raw OAuth access token as proof of login is a classic mistake.