Authentication is the process of validating a user's identity. Common authentication methods include knowledge-based (passwords), possession-based (tokens), and biometric-based (fingerprints, iris scans). Passwords are vulnerable to guessing and cracking, while biometrics have privacy issues. Passphrases are longer passwords but can be difficult to remember. 3D passwords combine multiple authentication factors by requiring users to interact with virtual objects in a specific sequence, making them more difficult for attackers to guess than traditional passwords.