In computing, we could implement C in machine-code, write a Lisp compiler in C, and then write an analogy-solving program in Lisp. Each of these systems defines its own set of symbols, syntax rules, semantic rules, and operations. Think of any program, e.g. Wordperfect: it gives you a basic set of capabilities which are quite unlike those of the PC's machine code. So we can also describe the functional architecture of each higher level: of C, Lisp, etc. These higher levels are often called virtual machines: ``imaginary'' machines implemented as software, not hardware. There's a very nice discussion of this in Chapter X of Hofstadter's Gödel, Escher, Bach (Penguin 1981: PSY KH:H 67).
Engineers like neat hierarchies because it makes thinking about the problem easier. It's an open question as to whether the brain works like this, and indeed whether any complete intelligence can. Perhaps, for true intelligence, the levels must all be horribly mixed: see discussion of evolved robots in my second AI and PP lecture.