Identity

Identity is the representation of a person, organization or system within another system.

The useful distinction is between who something is, how that identity is proved, and what that identity is allowed to do.

Authentication

Authentication answers:

How do we establish that this user, device or service is the identity it claims to be?

Common approaches include:

passwords
passkeys
security keys
one-time codes
certificates
federated login

Authorization

Authorization answers:

What is this identity allowed to access or change?

Authentication and authorization are related but different.

Sessions

A session lets a system remember an authenticated user across requests.

Common browser mechanisms include cookies and server-side session state.