Legion comes with a single Level 100 boost to any character of your choice. But if you can't, for some reason, decide which class you want to play, Blizzard is introducing a new feature called Class Trials which serves as a class introduction tutorial before you begin your Legion leveling experience. Two intros can be played, the Broken Shore and Artifact intro quest.
Boosting a character to Level 100 will place you into a short starting experience before you join in the Broken Shore experience. Class Trials allow players to try various characters at level 100 as a part of a limited tutorial experience within the Broken Shore and allows players to play beyond the tutorial portion to experience a certain class through to acquiring their Artifact weapon. The short tutorial experience focuses mainly on teaching class' core abilities. Players will be able to apply a boost token at any time to keep the character they used during the Class Trial. For both the Boost and Class Trials, players will start with pre-defined specs that can be changed at a later time.
No Demon Hunter Boost & Trials
Demon Hunters are ineligible for both boost and class trials. If you want a Demon Hunter, you'll have to level it until 100 (yeah Outland, Northrend, Azeroth, Pandaria, Draenor, it sounds horrible and having it experienced a gazillion times, I'd rather stick to dungeon leveling).
Below you'll find Class Trial demonstrations of certain classes: