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During the tutorial, you're automatically given your very first hero, Jaina Proudmoore, which represents the Mage class. This hero possesses magic attack Fireblast, which can be used against any target and can bypass units with taunt. This hero power can be used strategically to remove Divine Shield from an enemy or to hit one of your minions with Enrage to make them stronger. Despite of being a magic attack, Fireblast is not affected by spell damage. Malfurion Stormrage represents the Druid class in Hearthstone. This too focuses on magic attacks. Druids have the ability to shapeshift, a buff that gives + attack for one turn and + 1 armor that stacks and remain throughout the game.  Rexxar represents the Hunter class. Hunters possess the hero power Steady Shot, which deals 2 damage directly to the enemy hero. While it is very effective for bypassing enemy minions, the Hunter cannot use Steady Shot on any other target. Steady Shot is a reliable source of damage, and works with an aggressive approach to build pressure on the opponent.  Uther the Lightbringer, or Sir Uther Lightbringer, represents the Paladin class. Paladins have a unique ability, Reinforce, that summons a 1/1 Silver Hand recruit. This can be used together with cards that work well with more minions on the field like Raid Leader and Frostwolf Warlord.  Anduin Wrynn represents the Priest class. Priests are masters of healing and can heal themselves and their minions using their hero power. Priest deck card consist of minions that can heal and even buff.  Valeera Sanguinar represents the Rogue class. Rogues are backstabbing assassins known for their stealth, speed, and deadly efficiency. Rogues possess the hero power Dagger Master. Rogues can equip 1/2 dagger, which enable them to attack either a minion or hero.  Thrall represents the Shaman.  Shamans possess the hero skill Totemic Call, which can summon a random totem. Totems are helpful for survival or for aggressive play.  Every time you use the shaman hero power, it will randomly summon a healing totem, which heals all damaged friendly minions for 1 health at the end of your turn. Stone Claw totem is a summon with 0/2 with taunt. Searing totem is a minion that has 1/1, similar to silver hand recruit from Paladin's Hero Power. Wrath of Air totem is a summon with 0/2 + 1 spell power.   Gul'dan represents the Warlock class. This character has two hero powers, Inferno and Life Tap. Inferno summons a 6/6 Infernal while Life Tap draws a card, and in return, you will receive 2 damage.  Garrosh Hellscream represents the Warrior class. His hero power is Armor Up, which gives 2 Armor and can be used in every turn for 2 mana. It stacks and is useful for survival. The premier mechanic of the Warrior class is Enrage, which gives an attack buff to a creature when it takes damage, like Amani Berserker and Grommash Hellscream.

Summary:
Get your hero. Know the class levels.