These sections survey previous accounts of the causes (aitia) or principles (archai) of things and events. Since Aristotle has said in previous sections that genuine knowledge is knowledge of causes and principles--of why and how things come to be (and pass away)--this is pretty clearly central to his study.
As for the four causes, they are the familiar four, known to all novice Philosophy students and apparently treated more thoroughly in the Physics. The answer to the question "What is it?", regarding any thing whatsoever, would be answered by an account of the causes or principles that account for it being what it is. In order, then: there's material cause (of what is it composed?), efficient cause (who or what instilled motion in the material to arrange it as this thing?), formal cause (what is its essence [to ti en enai] or substance [ousia]? //at 988b36//), and finally final cause (for the sake of what does it exist?).
I'm especially interested in the concept of final cause, in keeping with the insistence of modern science that such a thing is thoroughly unacceptable. [You may imagine that I am here brilliantly reciting one of the many, many philosophical exposés which conclude that a) modern science is thoroughly mechanistic and b) that merely mechanistic explanations leave something out of our account of the world as it is actually experienced.] My sense is that those critiques are basically right about something--that we can't expect science, as we have come to understand it, to answer all our questions--but I personally don't expect that science would benefit much by trying to accomodate teleology. I take it to be an obvious point that we need more and better explanations of the world than the merely scientific. Imagine the mayhem that follows were some hard-nosed biologist to replies in empirical mode to the question "Why do you love me?"
At the same time, the typical proponent of scientific teleology is probably more or less mistaken about what Aristotle actually meant by final cause. I'd like to get a sense of what he did mean... and determine where there's room in our understanding of the world for something similar, some cognate term.
Reading these sections did somewhat restore my faith in Martin Heidegger's discussion of the four causes at the beginning of "The Question Concerning Technology," however. There Heidegger suggests that Aristotle considered the four causes not as pseudo-scientific attempts at explanation, but as the pieces of a different kind of explanation.
So "What is a virus?" gets one kind of an answer from microbiologists, and another from a knowledgeable non-scientist. Aristotle would probably recognize the microbiologist's answer as suiting some purposes by relying mainly on material and (a certain aspect of) efficient causation. But a non-scientist will take the question in a different way, closer to the Greek meaning of aitia. That word meant something like "answers to a question (as in court), what one can be held responsible to" (Randall 123). So a non-scientist, when asked "What is a virus?" will describe what viruses look like, based on images they have seen, and what they do to host organisms.
Those kinds of answers are basically first takes at formal and final cause, and they actually constitute half-decent answers to a reasonable question. Microbiologists of course consider these kinds of answers, too, at least in the initial phases of trying to identify a virus (Does it fit the familiar form of other viruses?) and in their basic concern to know anything at all about them (What's interesting is what this thing does to people).