Simply put, demonolatry is the worship of demons. Obviously, some people object to this. I think there's room for nuance. I've found that they aren't really out to hurt people (unless you ask them to, probably). That doesn't mean they can't be kind of intense, of course. You may notice that demons each have a court ranking. I don't do much with that personally, but it does influence my practice in so far as I treat them with a solemnity and respect across the board, regardless of specific rank. Some people address demons by their rank and name (i.e. "Earl Raum") but I don't really do that, personally.
Some demons are very clearly (literally) demonized versions of pre-Christian gods (Amon, for example). Some demonolaters and occultists lean into this and claim Kemetic deities or the Ancient Greek goddess Hekate to be demons, which is a tendency I strongly dislike. It's confusing for everybody, and I think these deities are best honored in their original contexts to give them the most respect. I am obviously also a Kemetic and Hellenic pagan, but I think even those who aren't those kinds of pagans should probably think about the wider implication of absorbing those deities into demonolatry.
Now, why demons, you may ask? I just like them, I don't know. I wasn't raised Christian, so I don't really have that strong of an aversion. Goetic demons (my main focus) can teach you a lot of different things -- from alchemy to liberal arts. They aren't evil beings solely out for death and destruction.
Some notes:Currently, these are demons I have any sort of interest in. I do not have experience with all of them. I may expand this list to be more comprehensive in the future - we'll see.