"Here, you see examples of four system calls - open, read, write and close. Don't fret over the details of the syntax; that's not important right now. The point is this: through these system calls (and a few others) the Linux kernel provides the illusion of a 'file' - a sequence of bytes of data that has a name - and protects you from the underlying details of tracks and sectors and heads and free block lists that you'd have to get into if you wanted to talk to the hardware directly. That's what we mean by an abstraction.