Creeps, more traditionally known as Creatures in other games, are every other unit on the board that is not a hero. Creeps have a noticeable sub-variant, Melee Creeps - which are the reinforcing creeps that are randomly deployed at the start of each turn.
Creeps are either played from card through paying mana and the associated card, or summoned through items such as Book of the Dead or Horn of the Alpha. When playing creep cards from your hand, they have the same restrictions as spells as you must have a hero of the creep's color in the current lane to play it.
Creeps are similar to heroes in that they have the three primary attributes - Attack, Armor and Health. However, they cannot equip items. (But they can be targeted by valid consumable items such as Healing Salve!).
Melee Creeps are the most basic form of creeps, and provide fodder both for the player and the enemy combatant. Melee Creeps are not played from the hand, but are instead summoned through players casting spells or at the start of a new turn.
List of Creep Cards