a computer system consists of several hardware and software components. the primary hardware componets of a typical computer system includ a CPU, main memory disk drives, a key board, a mouse , a display screen, a bus, and several other I/O devices.
the software components of a typical UNIX system consist of several of several layers: applications shell language libraries, system call interface, UNIX kernel, and performs all tasks that seal with allocation and deallocation of system resouces.
The shell and applications layers comprise twhat is commomly known as the application user's interface (AUI)
The  language libraries and the system call interface comprise the application programmer's interface (API)
A computer using the UNIX operating system can run many programs for the same user at the same time (multiuser and multitasking). It can also support many different users at once ( multiuser and time sharing).
To operate in a multiuser enviroment, a user must log on and log off the system, in order to maintain the security of the system and the integrity of the files of all the users. UNIX uses a text-based command user interface, which allows typing commands on a command line. These commands must be typed in the proper syntax