Authentication is the process of proving that a person or device is who it claims to be, using something known, something held or something inherent. A password, a card and a fingerprint are all authentication factors. Authorization, deciding what the proven identity may do, is a separate step that follows it.
Every access decision starts with a question: who is this? Authentication answers it, and only then can a system decide what that identity is allowed to reach.
The three factor types
Knowledge is a password or code; possession is a card, fob or phone; inherence is a fingerprint, face or iris. Two factors from two different categories are genuinely stronger, while two passwords, or a card plus the number printed on it, are not. Door readers use the same categories.
Physical and logical authentication
A door reader authenticates a credential and hands the result to an access controller. A login authenticates a person to software. The failure modes differ: readers face cloned cards and tailgating, logins face reused passwords and stolen sessions, and both suffer when enrollment is careless.