Following is from from the book The Linux Command Line. The full PDF tutorial is also available here:
A command can be one of four different things:
An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as
programs written in C and C++, or programs written in scripting
languages such as the shell, perl, python, ruby, etc.
A command built into the shell itself. bash supports a number of commands internally called shell builtins. The cd command, for
example, is a shell builtin.
A shell function. These are miniature shell scripts incorporated into the environ- ment. We will cover configuring the
environment and writing shell functions in later chapters, but for
now, just be aware that they exist.
An alias. Commands that we can define ourselves, built from other commands.
To identify the type of command, you can use
type - Display information about command type.
$ type ls
ls is aliased to `ls --color=auto'
$ type cd
cd is a shell builtin
To display the location of command, you can use
$ which info
$ which init
Hope this is helpful for new users to know basic about commands.