Helmets in Destiny are the second most effective piece of Armor a Hunter, Warlock or Titan can wear (behind Chest Armor). Besides protecting the player from damage, Helmets also grant bonuses to the players abilities. The bonuses can increase grenade throw distance, melee attack speed, melee and grenade ability cooldowns, give added Super Energy from killing enemies or bonuses when picking up Orbs of Light. The higher the quality of Helmet, the more bonuses it gets. The higher the level of the Gauntlet, the more Defense it provides.
In Destiny, Warlock Helmets are an Armor slot available to the specific Class. This Armor protects the head of the Warlock and comes in different qualities and styles. There are Basic/Uncommon/Rare/Legendary/Exotic Warlock Helmets that one can acquire while playing Destiny and from Uncommon on up, the Helmets begin coming with Helmet Upgrades which further benefit the player (beyond the Armor Value of the Armor).