The Eladrin are one of the three Eldar races, whose homeland is the Feywild nation of Auras. They are also sometimes called the "High Elves" by those who call all the Eldar peoples "Elves" (or by those classifying them based on the standard D&D Elven subraces).
Since the dawn of the age of mortals, these short and clever clinically immortal humanoids with pointy ears were ruled by King Kelvoras and Queen Avoras, but the King was slain in the Shatterwar by the orc Jamargar, mighty Lieutenant of the Varharran. The Prince has since stepped up as co-ruler alongside his mother, the Queen, but has thus far refused to officially take on the mantle of King.
These Eladrin have no relation to the Eladrin of 4th edition D&D (or other previous sources), whose name Demonac took without any of the substance.