Zombies have three different stats that determine how they act in battle:
Power is how much damage the zombie does to the enemy.
Life is how many hit points your zombie has.
Speed is how fast your zombie moves, which includes running toward your enemy, the frequency of attacks, and how fast the zombie's blue wait bar fills up during an invasion--unconfirmed but observations seem to point to that conclusion.
In this wiki stats are displayed as black bars, such as for the Garden Zombie will be: