Mob cards represent, obviously, Minecraft's Mobs and other creatures, or sometimes stuff that has health but isn't an structure, like in the case of the TNT card. Their icon is a Bone. Mob cards have a HP (health points) value that when depleted to 0, the card is knocked out and therefore, discarded. Most, if not all Mob cards also have abilities that can be used during the Battle Phase, and either inflict damage or cause another special effect, and may or may not require you to attach certain Items or Resources to the card.
Mobs are played in the Battle Front, above the Village which holds your Structures.