Honestly, it's a bit much for a single HN comment. But I've look around, and I found this, which upon first glance gives a good (and not too long) overview: http://www.varietysoftworks.com/jbaldwin/Education/single-le...

Crucially, it also describes the "single level store" that everything lives in. In short, the boundary between "main memory" or "hard disk" or other things is abstracted away, and you just have pointers to "things" instead. While in UNIX accessing some piece of main memory is fundamentally different from, say, opening a file.

Even though UNIX has a little bit of the opposite concept of trying -- but in my mind failing -- to represent as much as it can as a "file".