Night Elf Heroes
Primary Attribute: Agility
Mana Burn: Removes a set amount of mana from a target unit and then inflicts the same amount of damage on that unit.
Immolation: Surrounds the demon hunter in flames, damaging units adjacent to him.
Evasion: Makes the demon hunter harder to hit.
Metamorphosis (Ultimate): The demon hunter morphs into a demon, gaining extra hit points and a powerful ranged attack.
The demon hunter is a nimble melee fighter who can be a serious pain for your enemies to deal with. He does a lot of damage per attack and is fast-moving enough to be difficult for enemies to track down. You'll have to micromanage him a lot to keep him alive, however, since he's fairly weak, like the blademaster. His mana burn ability is excellent to use on spellcasting heroes like the far seer or lich, who depend on their spells to be effective. Immolation is also great, since it will cause bonus damage to any unit the demon hunter is attacking. Remember to turn it off when you're done fighting, though, because otherwise it will drain your mana until it's empty. Evasion is a self-explanatory passive ability that gives you a chance of dodging enemy attacks. Finally, metamorphosis makes the demon hunter much more powerful, boosting his hit points by 500 and letting him attack from a distance. The demon hunter is quite an imposing hero when leveled up and used properly.
Keeper of the Grove
Primary Attribute: Intelligence
Entangling Roots: Roots a target hero to the ground, causing damage for several seconds.
Force of Nature: Creates treants from nearby trees to fight for you.
Thorns Aura: Nearby units will cause damage to melee attackers when attacked.
Tranquility (Ultimate): Large area-of-effect healing spell that heals at a rate of 20 hit points per second.
The keeper of the grove is a great support hero for your army alongside a demon hunter or priestess of the moon. His entangling roots ability stops a unit in its tracks and damages it while it's trapped, and it is best used on enemy heroes. You can keep melee heroes from approaching your troops, and fleeing heroes can be trapped so you can surround and destroy them. Force of nature is another excellent ability to have in a battle, as it generates extra melee units for you with no supply cost. Thorns aura is a decent buff for your regular units that will passively help them fight back against melee invaders. Finally, tranquility is a seriously effective healing spell that will make your units very hard to kill in a battle. Be warned that the keeper must be stationary to keep casting tranquility--if he's disrupted, the spell ends.
Priestess of the Moon
Primary Attribute: Intelligence
Sentinel: Creates a ghostly owl that you can use to scout the map.
Searing Arrows: The priestess' regular attack gains bonus damage at a mana cost.
Trueshot Aura: Nearby ranged attackers gain bonus damage.
Starfall (Ultimate): Brings a rain of stars down on the enemy, obliterating it.
The priestess of the moon is probably the best hero to lead your ranged army with. She has a powerful ranged attack herself, making her good to group with archers. Her sentinel ability is useful at the beginning of the game, especially because you can scout out large portions of the map with it. Searing arrows is also an effective ability to use, as it makes the priestess' attacks do substantially more damage. Watch out, though, because searing arrows drains mana with every arrow fired. Don't let it drain all your mana if you can help it. Trueshot aura is another reason the priestess is good to have with units like archers and dryads, since it enhances their attacks. Finally, the best reason to use a priestess of the moon is for her ultimate spell, starfall. In short, starfall is an army killer. It's not simply a small blanket-of-effect area--rather, the stars seek out enemy units over a very wide area and do massive damage per hit. If you can cast starfall and keep the priestess safe for the duration of the spell, the enemy army's destruction is almost guaranteed.