Authentication is the process of validating and verifying the identity of a user or system trying to access a system or service before granting access. Authentication comes after the Identification process.
Authentication can be done with the following ways
What you know is where a user is authenticated with what they know like passwords, passphrase, keys etc.
What you have is where a user is authenticated with what they have like hardware/software tokens & fobs.
What you are is where a user is authenticated with what they are biologically which is biometrics like fingerprints, facial recognition, iris scans, hand geometry, retina scans, voice recognition.