The Fel Hammer is the Demon Hunter class Order Hall.
On the upper level, you will find many familiar NPCs, a Scouting Map for Missions and zone quest starts, and two outside landings (one of which holds an Illidari Gateway to return you to Dalaran).
If you've chosen the second tier Order Advancement Twisting Nether, you can use the Sanctum, Crystal to the right of the <Class Hall Upgrades> NPC to portal to the third, very bottom level of the ship, granting access a Demon boss fight each day.
You will find a cast of NPCs who should be familiar to you, by the time you are finished with your starting experience as a Demon Hunter.