Demon Hunter is the newest class being added to World of Warcraft for the Legion expansion. They are a hero class, which means they have their own introduction/tutorial and start at a higher level. You start off at level 98 and reach level 100 after the intro, but you are required to have a level 70 character on that server before you can make a Demon Hunter.
When you make a Demon Hunter, you must wait 1 week before you can do Legacy raids. This is to prevent people from farming them by just creating a Demon Hunter, deleting it, then creating it again.