Shell is a user program or it is an environment provided for user interaction. Shell is an command language interpreter that executes commands read from the standard input devices. Shell is not part of system kernel, but use the system kernel to execute programs, create file etc. Shell program is series of Linux commands. Shell script can take input from user, file and output then so screen.

There are several Shell available with Linux that include:
BASH (Bourne-Again SHell) :- BASH is developed by Brian Fox and Chat Ramsey at free software foundation. BASH most common Shell in Linux. It is freeware Shell.
CSH (C SHell) :- CSH is developed by Bill Joy at the university of California. The C SHell’s syntax and usage are very similar to the C programming language.
KSH (Korn SHell) :- KSH is developed by David Korn at AT & Bell labs.