Shell (computing)

An operating system shell is a user interface that enables the user to interact with and access the services offered by the operating system. The user gives commands to the operating system through its shell. 
In simple words - A shell is a program that accepts the input from users, and excutes program on theire behalf, by creating a new process and whatever the response from that perticular programs comes, which could be input, output or error, it shows back it the the user. 
Various shells.
There are various types of shells:
Examples of command line operating systems are UNIX and Disk Operating System (DOS). Examples of menu driven operating systems are the DOS shell. Finally, examples of graphical user interface (GUI) operating system are Linux and Microsoft Windows.