The shell is a user program that provides a command prompt on a terminal, and processes the typed commands.
There are various shells that can be used in Linux. Here I talk about the bash shell.
If your using a GUI such as Ubuntu then you just need to start a terminal and you will get a window with a prompt which is a $ for a normal user or a # for a root user, like this;
You are now using the "shell". It tells you that you are on the computer called "gold" in the home directory for "tom". Of course your account name and computer name will probably be different. Each account on any named computer has a directory allocated to it known as a "home directory".
To find out more about users and groups you can look at Users and Groups later.
To find out which shell you are using, type
echo $SHELL like this;
tom@gold:~$ echo $SHELL
if its the bash shell, which is the commonest, the system responds as above.
You can access the Shell Manuals directly from the terminal and so can be very useful.
There are three main types of function the file system provides through the shell;