In Linux and Unix, a shell refers to a program that is used to interpret the typed commands the user sends to the operating system. The closest analogy in Windows is the DOS Command Prompt. However, unlike in Windows, Linux and Unix computers allow the user to choose what shell they would like to use.
The original Bourne shell is named after its developer at Bell Labs, Steve Bourne. It was the first shell used for the Unix operating system, and it has been largely surpassed in functionality by many of the more recent shells. However, all Unix and many Linux versions allow users to switch to the original Bourne Shell, known simply as "sh," if they choose to forgo features such as file name completion and command histories that later shells have added.
The C shell, as its name might imply, was designed to allow users to write shell script programs using a syntax very similar to that of the C programming language. It is known as "csh."
TC shell is an expansion upon the C shell. It has all the same features, but adds the ability to use keystrokes from the Emacs word processor program to edit text on the command line. For example, users can press Esc-D to delete the rest of the highlighted word. It is also known as "tcsh."
Korn Shell was also written by a developer at Bell Labs, David Korn. It attempts to merge the features of the C shell, TC shell and Bourne shell under one package. It also includes the ability for developers to create new shell commands as the need arises.
It is known as "ksh."
The Bourne-Again shell is an updated version of the original Bourne shell that was created by the Free Software Foundation for its open source GNU project. For this reason, it is a widely used shell in the open source community.
Its syntax is similar to that used by the Bourne shell, however it incorporates some of the more advanced features found in the C, TC and Korn shells.
Among the added features that Bourne lacked are the ability to complete file names by pressing the TAB key, the ability to remember a history of recent commands and the ability to run multiple programs in the background at once.
It is known as "bash."