>>27581075IVs are a bonus to each stat, given when a Pokemon is born, ranging from 0 to 31 for each stat. These are a direct bonus to the stat, from 0 to 31 stat points, once a Pokemon is at Lv100. So a Sandslash with no EVs could have an HP stat of 260-291 at Lv100, depending on the IV for its HP.
EVs are gained through battle, each Pokemon having their own value when defeated (for example, Zubat give one EP, or Effort Point, for Speed). Every four EVs you gain grants an additional stat point, and Pokemon can have a maximum of 510 total, in all stats combined, before they can no longer gain more, and only 252 in any single stat. This translates to a total stat bonus of 127 across the board, and 63 in any single stat at maximum.
