EVs (Effort Values) are one of many hidden values that affect your pokemon's stats. There's also nature values (dealing with the 10% bonuses or penalties that different natures offer to different stats), level value (just your pokemon's level). and IVs (Individual Values).

EVs and IVs are most influential towards your pokemon's stats. Only EVs are easily manipulated as IVs are randomly generated when a pokemon is caught/hatched. For more detailed explanations check out the subcategories listed on this page.