Multi-user
& Multi-process
Many
people can use one machine at the same time.
File
& Process
Data,
directory, process, hard disk etc
(almost everything) are expressed as a file. 
Process
is an running program identified by a unique id (PID).
Important
Directories
/bin     This contains files that are essential for
correct operation of the system. These are available for use by all users.
/home
This is where user home directories are stored. 
/var   This directory is used to store files which
change frequently, and must be available to be written to.
etc    Various
system configuration files are stored here. 
Commands
 su  switch user
passwd   change password
 useradd  create new user account
 userdel  delete user account
 mount  mount file system
 
HI MR HAIDAR CONGRATULATION
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