The shell interprets commands with the operating system kernel. The beauty of Unix is that hundreds of people can access one box at once and each one can run a number of programs, a separate shell opens for each log on that occurs, and each user can have a completely different 'environment' setup, different colours, priviledges, file and directory access and different shell. Every Unix command or filename is case sensitive, unlike DOS, this is the most common error to be aware of. This is so that one user does not hog one program, many people can access it. Such a program is called a 'daemon'.
User names can change without changing user permissions.
A user may be a member of several groups each having different permissions. Comment containing more detail on the user if desired. User's home directory. The shell to be used by the particular user. Nowadays, when you are confronted with a Unix box you will come across an X-windows interface.