Each type of Pokemon has a base set of attributes for Stamina, Attack and Defence. Each specific Pokemon is "born" with it's own IV (individual values), that further increase these stats by 0-15 extra points. You can not change the IV's, not all Pokemon are born equal.
These IV's make small differences in the final power of a Pokemon. For example, the base attributes of a Charizard are 212, 182, 156. This would be the "worst" charizard available where all IV's are zero. With max IV's, a Charizard would have 227, 197, 171. Each of these Atk, Def and Stam values for Charizard are only 7-10% better.
