What is a command shell?
A program that interprets commands
Allows a user to execute commands by typing them manually at a terminal, or automatically in programs called shell scripts.
A shell is not an operating system. It is a way to interface with the operating system and run commands.
What is BASH?
BASH = Bourne Again SHell
Bash is a shell written as a free replacement to the standard Bourne Shell (/bin/sh) originally written by Steve Bourne for UNIX systems.