Health points (HP for short) shows how much damage the unit can take without dying. Usually heavier and bigger tanks have more health points than smaller ones. Unit's longetivity can be affected by various sources, like resistance and shields.
The damage value from in-game tooltip shows how much damage a unit does by a single hit. Since different units have different attack intervals between shots, currently this value doesn't represent the true attack power of the unit. To calculate the actual damage per second (DPS for short) value, we need to divide damage per hit by attack interval
DPS = Damage / Interval
For your convenience this wiki shows the actual damage per second value in "Damage" fields.
Area-of-effect (AOE) and normal attacksEdit
Normal attacks damage only one unit, which they are targeted to. Area-of-effect attacks deal damage to all units in a certain area. There are different types of resistance against these attack types.