Alfheim is one of The Nine Realms and exists on the same landmass as Asgard. It is the original home of the Elves, descendants of the Fae Realms and decedents of the Tuatha de Danaan. Much of Alfheim borders the Fae Realms and it is not a great distance to Otherworld via those routes.
Two races occupy Alfheim-- the Light Elves and the Dark Elves, refugees of the ancient Seelie and Unseelie courts. The Dark Elves were banished from Alfheim after Ragnarok and took up residence near the Dwarven dimension of Nidavellir.
The capital of Alfheim is Ljosalfgard, and it is ruled by Queen Aelsa Featherwine. Alfheim owes allegiance to Asgard and is closely allied with them.