User authentication is the process of verifying the identity of a user before granting access to a system or application. This ensures that only authorized individuals can access sensitive information or perform specific actions. Common methods of user authentication include passwords, biometric data like fingerprints, and two-factor authentication, which adds an extra layer of security. When a user attempts to log in, the system checks their credentials against stored data. If the information matches, access is granted; if not, the user is denied entry. This process is crucial for protecting personal data and maintaining the integrity of systems, such as websites, mobile apps, and online services.