Everything is a File

"On a UNIX system, everything is a file; if something is not a file, it is a process."

"Everything is a file" is a core design principle of Unix (and hence Linux) architecture. It can be taken to mean two things

Everything is a stream of bytes
Everything is a stream of bytes you can read and/or write to. Not just documents but also storage devices like a CD-ROM in fact almost any device. This is in keeping with Unix philosophy which favors simplicity, universality and plain text. This means that you can treat devices and other parts of the system just like you would any other text file. To demonstrate this plug in an external mouse and type sudo cat /dev/input/mice in the terminal and move your mouse around. It might become clearer if you pipe the output through hexdump sudo cat /dev/input/mice hexdump