Loyalties, mostly. Mab rules the Unseelie Court, while Oberon and Titania rule the Seelie. The courts divided after Mab and Oberon had a falling out. Family squabbles and all that. Unseelie are primarily nocturnal creatures who favor the cold, while many Seelie are comfortable in summer and sunlight. Me, I'm really too old for it to make all that much difference either way.