In DnD 5e, Eladrin are elves that have Fey Ancestry. They are long descendants of fey creatures and retain some characteristics of the Fey. They are considered humanoid creatures (elves) for rules purposes.  In DnD 3.5, there was a distinction between Celestial Eladrin and Eladrin. They are a different subspecies of Eladrin for a different version of DnD and were also considered Elves descended from the Fey, but also with Celestial ancestry as well.