Aristotle's God is in many ways a matter which is widely open to conjecture. There are a number of ways you can actually look at it. Anyway? what is Aristotle's God? To understand this, you have to understand Aristotle's logical track. If you do not follow the track, you just come up with a very unsupported axiomatic statement that no one would be able to understand in any event. I could just say "Aristotle's God is the unmoved mover," but then where does that leave you. To me, I would be more confused. The sources I use in this interpretation are; Aristotle's Metaphysics and scholarly work of two dominant views, Paul Natorp and Werner Jaeger.

A? First, it is important to understand Aristotle and the context in which we discuss his "God." Book Alpha of the Metaphysics underlines the primacy of "first philosophy" as a "sought-for science." This is important to note because in book Gamma, the sought-for science is going to be the examination of what Aristotle calls "being qua being" (which essentially means "substance as substance in itself"). So keep in mind that God and substantial onotology are very closely linked.

B? Next, Aristotle mentions theological aspects in particular. In Book Epsilon that all thinking is going to be divided into three separate parts? theoretical, practical, and artistic. (1025b25). Of the theoretical branch, there are three essential parts; mathematical, natural, and theological. Not only is this important to note because it denotes a possible theological tendency, but following that previous classification, Aristotle identifies the "sought-for science" as a theoretical/theological branch.

C? The next thing to consider is Aristotle and the role of Paronomy. Paronomy is essentially to be called something in relation to some one thing. So take for example an elaboration of Paronomy in Book Gamma; "Health is both health in itself and the principle cause of being healthy. It is the first healthy thing. (1003a33)" According to Paul Natorp, this is meant to link Aristotle's ousia as substance. So substance is a being in itself (being qua being) and a principle/cause for all that is in other categories (qualities, quantities, etc.). I would mention that this argument leads into what is perhaps the best support for ontology yet devised(and frequently debated topic on the forum)? but that is another topic. Its important to note paronomy because of the fundamental argumentation that's used? which is that a thing is to be considered via another thing. So When Aristotle says "the hand is the tool of tools," it is important to consider that tools are considered tools only by what activity is performed by the hand. In relation to God, this reasoning is critical to show how nous (extends to substance?) is the eidos (form of forms).

Ok, recap. So you have A, God is relative to substance, B, Aristotle's admits a theological point, and C, Paronomy for Aristotle begins to show a sort of cause, effect, and a precursor to that causes' cause beginning to take shape.

So now when you get to Book Lambda, you get to Aristotle's conception of God. The introduction Book Lamda (widely regarded as Aristotle's address to Theology (Jaeger) in Metaphysics) states that, "A thought thinks on itself because it shares the nature of the objects of thoughts. (1072b20)" As you go further along, you find out that God is the first substance among substances? the first being in itself as well as the substance that all other substances rely on (see how the description on paronomy was useful now?). But this is just the fundamental conceptions of God as the (to be later determined) "unmoved mover."

As far as what it is to be an "unmoved mover," Aristotle asks (in Lambda 6-7) how motion comes about if there was nothing moving it. (1071b28) However, there is something which is not moved according to Aristotle. A "being purely actual," a being that cannot be anything else except what it is. Because if we rationally think about it, to move (Aristotle's locomotion), a movement is induced by a first movement? or mover. So the "first mover" has to be necessary, and if necessary is the primary principle and integrally important. It is on this first mover? the unmoved mover? that heavens and nature depend. (1072b5)

So in short, Aristotle's God is the unmoved mover. There is a lot more to it actually, but this is a rough outline of what the essential nature of God is for Aristotle. Now as far as what makes this God worth having, could you imagine a universe in non-motion without causation or necessity?

God The former of the form of forms;
Before beginning patent use ending invention.