Question. Are silver and magic swords the only swords that can work on ghosts? For example are ebony and glass swords useless?

I know that steel and iron won't work but how about the higher ones, like glass and so on?

Yes, Glass, Ebony and Daedric work as well.
Also any enchanted weapon, even if it's Iron.

Why does the undead have a resistance to regular weapons in Oblivion? but in Skyrim there are no resistances to weapons, well....only with the exception of the Lord Stone.

Skyrim was the first game to do that! Oblivion, Morrowind, and I believe Daggerfall as well you needed silver or above to be able to harm ghost.

I thought it was only ghosts that resist normal weapons, and maybe vampires. I assume Skyrim is the same. (I don't really remember, and by the time I ran into anything even related to a ghost I had good enough gear.

Silver or better works against ghosts IIRC so silver, dwever, glass, ebony, Daedric, elven? I'm not sure about elven.

If you have SI then Madness and Amber works too IIRC.

Hate to correct you guys, but it's silver, daedric, or any enchanted weapon for ghosts. There are a handful of non-enchanted weapons that can harm ghosts, but they're few and far between.

Yeah. It was Morrowind that did the whole "anything a grade higher than Silver" that bypassed normal weapon resistance.