A User ID is a unique identifier assigned to an individual user within a system or application. It helps distinguish one user from another, ensuring that each person's data and activities are kept separate. User IDs can be in the form of numbers, letters, or a combination of both, and they are often required for logging into accounts on platforms like social media, email services, or online banking. User IDs are essential for maintaining security and privacy in digital environments. They allow systems to track user behavior, manage access rights, and personalize experiences. When users create accounts, they typically choose a User ID or receive one automatically, which they use alongside a password to access their accounts securely.