The shell (also known as "command-line interpreter" or CLI) is a special application which interacts with the user and the system, thus creating an interface between the two.
The shell takes care of executing processes, managing the terminal, and interacting with users.
As long as the user is logged in, at least one shell is running.
To know what the current shell, use the command echo $SHELL.
$SHELL is an environment variable. The list of environment variables
can be listed with env or printenv (bash, sh, zsh) or setenv
(csh, tcsh).
Environment variables control the behavior of programs and shells.