A file is a collection of binary data that represents information. There are basically two types of files, text or binary. Is worth it to point out that the binary contents are interpreted by software or hardware. 

Keep in mind that you will probably do not use most of these locations during your journey of learning Linux. It is more relevant to know this, for System Administrators and Kernel Developers. 

A useful command that will help you to find out where a tool or program is stored in the which command. 

which ls

If we wish to explore the directories and see "what is in there", we can do that with the command ls. Here there is an example of how this command works:

(To visualize the directories on the root directory)

ls /

A different way to use the command ls that it may help you during your working days is 

ls -l /

With this command as you can see below, we can now visualize the directory root vertically and we have also more information.