Unlike many other operating systems currently available on the market, Linux supports the concept of multiple users and groups. A user is someone who has a unique identifier on the system, both a name and a number. This information allows the system to control how access is granted to the system and what the person might do after he or she has been allowed in. Users'activities are tracked by the system using their unique user identification number.
The difference between login and username
You will often find that people use the terms login and username interchangeably. Within most contexts, they mean the same thing.
A group is a collection of users. Every group also has a unique identification ...