Shell Basics

Shell is a command prompt that accepts the commands you type there and basically makes the computer do what you tell it to.In the old days,shell was the only user interface available on a Unix-like system such as Linux.Nowadays, we have graphical user interfaces (GUI) in addition to command line interfaces (CLI).Linux was also designed as a multi-user operating system.which means that you can have more than one shell running at the same time.
On most Linux systems a program called BASH an enhanced version of the original Unix shell program written by Steve Bourne acts as the shell program.

Different types of Shells

•  Bourne Shell (sh)
•  C shell (csh)
•  Tc shell (tcsh)
•  Korn sell (ksh)
•  Bourne again shell (Bash)

To find the available shells in your linux machine type
# cat /etc/shells

To view which shell is using
#echo $SHELL

Some examples to try in terminal 

$ echo D*
Desktop Documents

$ echo [[:upper:]]*
Desktop Documents Music Pictures Public Templates Videos

$ echo /usr/*/share
/usr/kerberos/share /usr/local/share

$ echo $((8 + 8))
16

$ echo {A..Z}
A B C D E F ... Z

$ echo "$(cal)"

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Terminal  is a program that opens a window and lets you interact with the shell. There are a bunch of different terminal emulators you can use.

Shell script  is a series of command written in plain text file.The shell reads this file and carries out the commands as though they have been entered directly on the command line.