They look like ghosts?
Don't agree with the fact that they can be decapitated and killed with any weapon though. Was good in OB and MW in that you needed a magic or silver weapon.
I remember getting killed by ghosts in Oblivion because I didn't have magic/silver weapons or magic spells to use against them. Later on, that wasn't a problem.
Bethesda should have kept that part in Skyrim, rather than have ghosts be simply spectral versions of regular enemies.