Visualize the operating system as an egg with a shell around it for protection. The shell provides the interface to the outside world. The Unix shell interprets commands entered by the user. The shell reads in the commands you type and interprets what they mean. Unix can be run with several different shell programs.
Users can select an existing shell program or write their own.
It is a variation of the c-shell from Berkeley. These shells are all very similar and the features mentioned here apply to all of them. It reads in the command and decides what to do with it. Most Unix commands are just names of program files that are loaded and executed for you. If you see the message 'Command not found', the shell is telling you that it could not find a file by the name you typed.