Shell

Shell is a command-line interpreter that provides text user interface for an Operating System.

User can use shell inside Emacs, with shell mode, eshell or term.

shell mode

in this mode, Emacs run an inferior shell, with I/O through buffer (default to *shell*).

eshell

in this mode, Emacs emulate a shell. It is writen by Emacs Lisp.

term

in this mode, Emacs emulate a terminal, then Emacs invoke a shell that you choose, then link to the terminal.