SSO lets users authenticate once and access multiple applications:
Identity Provider (IdP): Central authentication service (Okta, Auth0, Azure AD)
Service Providers (SP): Your applications that trust the IdP
Flow: User accesses App A, gets redirected to IdP
User authenticates at IdP
IdP creates session, redirects back with token
User accesses App B, redirected to IdP
IdP sees existing session, immediately redirects with token
SAML and OIDC are common SSO protocols. OIDC is newer and simpler.