In computing, a shell is a user interface for access to an operating system's services.
In general, operating system shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer's role and particular operation.
It is named a shell because it is the outermost layer around the operating system.
Powershell, Bash, ZSH, the MINGW git shell for Windows, and the Win32 shell you use when you open CMD.exe in windows are all shells you might have encountered.
Great wiki about different shells:
We're going to be focusing mostly on Bash and ZSH
Prompt Statement (PS1, PS2)