In computing, a shell is a piece of software that provides an interface for users. Typically, the term refers to an operating system shell which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web browsers and email clients that are "shells" for HTML rendering engines. The name 'shell' originates from shells being an outer layer of interface between the user and the innards of the operating system (the kernel).
Operating system shells generally fall into one of two categories: command line and graphical. Command line shells provide a command line interface (CLI) to the operating system, while graphical shells provide a graphical user interface (GUI). In either category the primary purpose of the shell is to invoke or "launch" another program, however, shells frequently have additional capabilities such as viewing the contents of directories.
The relative merits of CLI- and GUI-based shells are often debated. CLI proponents claim that certain operations can be performed much faster under CLI shells than under GUI shells (such as moving files, for example). However, GUI proponents advocate the comparative usability and simplicity of GUI shells. The best choice is often determined by the way in which a computer will be used. On a server mainly used for data transfers and processing with expert administration, a CLI is likely to be the best choice. On the other hand, a GUI would be more appropriate for a computer to be used for image or video editing and the development of the above data.
Modern versions of the Microsoft Windows operating system use Windows Explorer as their shell. Explorer provides the familiar desktop environment, Start menu, and task bar, as well as the file management functions of the operating system. Older versions also include Program Manager which was the Shell for the 3.x series of Microsoft Windows.
Many individuals and developers dissatisfied with the interface of Windows Explorer have developed software that either alters the functioning and appearance of the shell or replaces it entirely. WindowBlinds by StarDock is a good example of the former sort of application. LiteStep, GeoShell and BB4Win are good examples of the latter.

Graphical (GUI) shells
Non-Unix shells:
Shells for programming languages:
Unix shells:

AmigaOS - Amiga CLI/Amiga Shell which functioned as alternative to the Amiga GUI called Workbench
Basic-Plus - RSTS/E
CCP - console command processor of CP/M
COMMAND.COM - shell for DOS
cmd.exe - shell for OS/2 in text mode and for Windows NT and later Windows operating systems
Commodore DOS Wedge - an extension to the Commodore 64's BASIC 2.0 that included shorthand for common disk operations
DCL - the standard shell for OpenVMS, deriving from versions created for earlier DEC operating systems
DDT - PDP-10 debugger from DEC used as a command shell for the MIT Incompatible Timesharing System
DROS - J2ME Platform based DOS like shell for smart phones.
iSeries QSHELL - 'UNIX style shell' on the IBM OS/400
Macintosh Programmer's Workshop - old command line environment used for software development on the classic Mac OS
Microsoft BASIC - the primary operating environment for a number of older 8-bit computer systems
Rexx - IBM's scripting language
Windows PowerShell - an object-oriented replacement for cmd.exe (formerly known as Monad or the Microsoft Shell (MSH))
Windows Recovery Console - feature of the Windows 2000, Windows XP and Windows 2003 operating systems
YouShell - JavaScript based command processor of YouOS
4DOS, 4OS2, 4NT - shells for DOS, OS/2, and Windows NT written and sold by JP Software. The Take Command programs were compatible shells requiring and enjoying a GUI.
JavaScript shell [1]
JavaScript shell bookmarklet for IE [2]
Python's standard interpreter can be invoked in a shell mode
Interactive Ruby Shell interactive version of ruby
GMLCMD - GML Shell
BeanShell - Shell for Java
Bourne shell (sh)

• Almquist shell (ash)
  Bourne-Again shell (bash)
  Korn shell (ksh)
  Friendly interactive shell (fish)
  C shell (csh)
  
  
  
  • TENEX C shell (tcsh)
    Es shell (es)
    rc shell (rc) - shell for Plan 9 and Unix
    scsh (Scheme Shell)
    Z shell (zsh)