Each Pokémon has Combat Power (CP) and Hit Points (HP).
All Pokémon start out with full HP at capture, but HP can be depleted during battle.
Each individual Pokémon is assigned CP at capture, which indicates how well that particular Pokémon will perform in battle. Not all Pokémon have the same CP - for example, if you have caught two Pikachu, they will not necessarily have the same CP.
As you gain XP and become a higher level Trainer, the CP of the Pokémon you capture will generally be higher. However, some types of Pokémon will naturally always have lower CP.
You can Power Up your Pokémon to increase their CP and HP.