There are two types of card in Zombie Battleground: Zombies and Items.
The one feature that they share in common is their cost in goo, listed in the top right corner.
Zombies are the most numerous type of card, and act as the basic units of combat on the battleground. Zombies will fight for their overlord as directed by the player, and can attack enemy zombies or the opposing overlord. Zombies have 5 important traits in addition to their goo cost: faction, rank, type, attack, and defense, all of which can impact how they interact with other zombies. Additionally, many zombies have extra effects that are described by text on the card, which may include keywords as shorthand for abilities and trigger conditions.
Items are single-use cards that can be played to activate some sort of effect, as described in the card's text.