Damage per Second is probably the most important factor in managing your Zombie killing. The word "Damage per Second" itself literally translates to "How much damage is done over 1 second". The formula can be found here:

DPS = Damage done to Zombie in 1 shot ÷ Time between shot intervals

For example, a Peashooter does 1 damage per pea, and each pea shoots every 1 second.

Thus, the peashooter has the DPS of 1 ÷ 1 = 1.

A Cabbage-pult does 2 damage per shot, but fires every 2 seconds.

Thus, the DPS is 2 ÷ 2 = 1, even though the Cabbage-pult does 2 damage per shot.

How about the Melon-pult?

DPS = 4 ÷ 2 = 2 per second, but of course one of its hits does a mighty 4 damage.

Now, the DPS can be used to see how along a plant takes to kill a Zombie.

Formula is Time to kill a Zombie = Zombie's health ÷ DPS of Plant

For example, the time for a peashooter to kill 1 Zombie would be:

10 ÷ 1 ≈ 10 seconds to kill a Zombie.

Notice I used the ≈ sign as it is not always right, as the distance between the plant and Zombie also applies, but if left on its own the Peashooter WILL kill a Zombie in around 10 seconds.

How about the time for 2 Repeaters to kill 2 Coneheads?

(28 + 28) ÷ (2 + 2) ≈ 14 seconds.

Also, some plants have varying DPS, like the Kernel-Pult and Cob Cannon. The Kernel-pult, while firing a Kernel, does an average of 1/2 DPS, as it fires every 2 seconds and each Kernel does the damage of a Pea. While firing butter, it does an average of 1 DPS, as butter does the damage of 2 Peas, also stalling the Zombie. The Cob Cannon reloads every 36 seconds (if you count the time to fire), and each Cob has been estimated to do about 100 damage, so the DPS of a Cob Cannon is 100/36 = 25/9, which is about 2.7.