A shell is a command line interpreter. Its a special program which takes input from standard input devices, convert it to machine language and send to OS. After processing by OS, send result back to shell. Shell again translate it to human readable format and send results to output devices.
To Get all available shells in your linux machine check /etc/shells file.
# cat /etc/shells /bin/sh /bin/bash /sbin/nologin /bin/tcsh /bin/csh /bin/ksh /usr/bin/ksh /bin/pdksh
If you want to switch to another shell, Just type the shell path as shown above.
# /bin/tcsh or # /usr/bin/ksh