Creatures deal damage to creatures and players in the form of combat damage. When they deal combat damage, they deal it equal to their power. Some spells and abilities also deal damage to players. When the damage hits a creature, if it reduces the toughness to 0, the creature dies. If it deals the damage to a player, the player loses that much life. Giving a creature -1/-1 with Infest or other similar cards is not damage, and therefore cannot be prevented with cards like Shieldmate's Blessing.
