A shell is basically an interface to the operating system. A shell contains a programming language by which you can interact with your system. Through shell, you directly communicate with your OS.
Introduction to Bash
Bash stands for "Bourne Again Shell" and was developed by Brian Fox in 1989. It's called "Bourne Again" because it replaces the popular Bourne Shell which is "born again" through Bash. Bash is a standard shell for Linux distributions.