A shell is a piece of software that provides an interface for users to an operating system which provides access to the services of a kernel. The name shell originates from shells being an outer layer of interface between the user and the innards of the operating system (the kernel).
In operating systems shell generally fall into one of two categories: command-line shell and graphical shell. A command-line shell provide a command-line interface (CLI) to the operating system, while a graphical shell provide a graphical user interface (GUI).  The primary purpose of the shell is to launch another program;  however,  shells frequently have additional capabilities.

The Z shell (zsh) is a Unix shell that can be used as an interactive login shell and as a powerful command interpreter for shell scripting. Zsh can be thought of as an extended bourne shell with a large number of improvements, including some of the most useful features of bash, ksh, and tcsh.

The first version of zsh was written by Paul Falstad in 1990 when he was a student at Princeton University. The name zsh derives from Zhong Shao, then a teaching assistant at Princeton University. Paul Falstad thought that Shao’s login name, “zsh”, was a good name for a shell.

Features of note include:
Programmable command line completion that can help the user type both options and arguments for most used commands, with out-of-the-box support for several hundred commands
Sharing of command history among all running shells
Extended file globbing allows file specification without needing to run an external program such as find
Improved variable/array handling
Editing of multi-line commands in a single buffer
Spelling correction
Various compatibility modes, e.g. zsh can pretend to be a Bourne shell when run as /bin/sh
Themeable prompts, including the ability to put prompt information on the right side of the screen and have it auto-hide when typing a long command
Fully customizable
Attesting to the sheer size of this shell is the first sentence of the shell’s manual page, which reads “Because zsh contains many features, the zsh manual has been split into a number of sections”, and then goes on to list thirteen items.

Windows Shell is the most visible aspect of the Microsoft Windows line of operating systems. The shell is the container inside of which the entire user interface is presented, including the Task bar, the Desktop, Windows Explorer, as well as many of the dialog boxes and interface controls, but also describes the past shells, like MS-DOS Executive and Program Manager.

The default Windows shell is called Explorer (confusingly, the same as the MS file browser) ? this is the program that determines the look of your desktop, i.e. it creates the task bar, system tray, start menu etc.

Windows 1 and 2 – Early UI
Just after the PC hit the market (August 1981), a project named “Interface Manager” started. It was renamed to “Windows” because the programmers talked very much about the zones called “windows” on the screen. Rowland Hanson, the head of marketing at Microsoft, convinced the company that the name Windows would be a more appealing name to consumers. The first Windows pre-version was presented in November 1983. It used Word for DOS-like menus at the bottom of the screen. The 1.0 version (it was numbered 1.01; it is rumored that version 1.00 was actually released but quickly pulled due to a severe flaw), released in November 1985, used pull-down menus like the early Macintosh System 1.x (Microsoft actually licensed GUI elements from Apple). The shell was a file manager (not a program manager) called “MS-DOS Executive”. Applications could be launched from the MS-DOS Executive which minimized itself. The minimizing (called “iconing”) was done by transforming the windows into an icon which was placed at the bottom of the screen, in a special minimized windows zone. The maximizing (called “zooming”) could extend the window over the minimized windows zone. Windows could not be overlapped, but they were instead “tiled”. As a result, two windows could not be “zoomed” at the same time.

Windows 2.0 was an interface-based release. The new window controls were introduced with this release, with the new “minimize” and “maximize” terminology. Windows could be overlapped and the minimized window icons could be moved freely on the desktop.

OS/2 1.x
As of version 1.1, launched in 1988, the new OS/2 operating system from IBM introduced a new GUI, called the Presentation Manager. The default shell (a program) was a program manager (not a file manager like in Windows 1.x and 2.x) called “Start Programs”. Versions 1.2 and 1.3 renamed “Start Programs” to “Desktop Manager”, added 16-colour icons support and many more.

Windows 3.x, NT 3.x – The First Revolution
Windows 3.0, introduced in May 1990, inherited the OS/2 GUI. The new “Program Manager” was a simple “front end” where the “groups” and icons had no relation to the actual file system. A background could be put on the desktop, and the window controls were redesigned. The buttons were all in 3-D appearance (the windows weren’t). As a result, the Windows 3.0 operating environment was a success. Later versions of Windows 3.x introduced Screen Savers.

The new operating system from Microsoft, “Windows NT”, featured the same GUI in the first version (3.1), like Windows 3.1x.

Windows 95C, 98 – “Nashville”
The growing popularity of the World Wide Web forced Microsoft to release its own browser, dubbed “Internet Explorer” which was based on technology licensed from Spyglass. In early 1996, Netscape announced that the next release of its browser, Netscape, would completely integrate with Windows and add a new shell, codenamed “Constellation”. Microsoft started working on a similar Internet Explorer release, codenamed “Nashville”. Internet Explorer 4.0 was redesigned and resulted in two products: the standalone IE4 which replaced the Windows shell with a new “Active Desktop” shell and the future Windows releases, like Windows 95C and Windows 98, which integrated Internet Explorer and Active Desktop in the shell.