On Unix and Linux a "file" is anything you can write data to or read data from. It includes not only stored data but also data communication channels between physical systems or processes.
Therefore hardware devices device like a disk drives, memory sticks or printers, are just treated as files in the file system.
There are also special files called "directories" which contain references to other files. Directories can also contain the references to other directories as well as files. Although it isn't physically the case we often think of directories as folders that contain other folders and files. The filesystem normally does not allow files or directories to be referenced in more than one other directory. (Links are an exception to this.)
The result is a tree like structure where there is a root directory that contains directories and files, where the directories contains directories and files and so on and so on.
Files and directories in the file structure can be refered to by their paths. So for example;
/home/tom/pictures/itsme.jpg
is the path to the file itsme.jpg. Every user on a linux system has their own "home directory". I am a user called "tom" and in my home directory I have a pictures directory containing a picture called itsme.jpg.
When you mount a device like a disk drive, memory stick or printer, in Linux, it is the process of allocating it a position in the file structure so that it can be referenced and so made use of.
The first partition on a hardrive could have the path /dev/hda1 the second partition path would be /dev/hda2.
The most obvious way for you to gain access to the file structure is through the "shell". The shell is the process that provides the user with the Command Line Interface on a Terminal. The Shell because the shell is your interface to the file system.