Shell is a program that takes your commands from the keyboard and gives them to the operating system to perform. Some common types of shells are listed below:
> /bin/bash
> /bin/ksh
> /bin/csh
> /bin/sh
> /sbin/nologin
etc…

How/command to find out the current shell ?

# echo $SHELL

Example:

[[email protected] ~]# echo $SHELL
/bin/bash

Related Post:
How to change the shell for an already existing user