What is a god?

I see names like Thartaag, Z'en, Ruptga, etc., but I'm not entirely sure what they are. I assumed they were different names for Daedra / Aedra. Is this the case? Or are they entities on the same ladder as the Tribunal?

I think that most of them are alternative names. However, it is possible that some of them are lesser Daedra/ Aedra (nothing can exist without at least one worshiper)

there are many, many things which could be considered a "god." Some are Daedra, others Aedra. There are also Et'ada who are neither (like the Mnemoli), and the Earthbones, both of which could be considered gods. Beings who have achieved incredible power by somehow tapping into the power of others (like the Tribunal and the King of Worms) can be called gods. people who get enough worshipers can also be considered gods, though they may not actually have any godly powers. Its a tricky subject, with no clear answer.

of the gods listed in Varieties of Faith, most are variants of Aedra, with most of the rest being culture heroes.

Morihaus, Tiber, Reman, Mannimarco, Vivec and Tosh Raka are all either demigods or former mortals-turned-god, and I suspect the Yokudan pantheon (which has gods for everything) is probably made up of more than a few of these types.