Youkai (along with a lot of other words that are almost synonymous like Ayakashi, Obake, etc,) is just the catch-all term for any Japanese spirit which is (usually) not a "kami". That's the short version, though it gets more complicated when you boil it down with some of the linguistic and semantic stuff but that is a good enough working definition.
There's not a lot you can say about them in general beyond that as they range from benevolent, to neutral, to malignant in their attitudes to humans and you really need to get specific about which ones you're talking about to say more. The one thing is that like most of these kinds of things, you need to take a more transactional approach to dealing with them if you want something out of the relationship(if they're the sort that are even interested in having one), so giving offerings in the right way is a good way to go.
They aren't that different from the spirits of other places when you strip it down to the old legends. Elves, the fey, it's kind of all different words for basically the same things. Compare the Zashiki-Warashi to the Domovoi, Kobold, Brownie, so on so forth and you'll see what I mean.