You place a sigil stone on the altar, which you can get from the Ritual Spell: Conjuration quest. I haven't actually tried summoning anything yet, but if the book on the table near the altar is to be believed you can summon a whole host of randomness, including but not limited to goats.
With the sigil stone, it allows you to summon higher level daedra and turn ebony items into their daedric counterparts. --- Now, if I could somehow play Skyrim while watching ponies, my life would be complete. //NVBM//