+P is ammunition loaded to higher than standard pressure. Ammuniton that is loaded to +P specs generally produce higher velocity and greater energy. This may or may not, equate to greater "stopping power." +P+ is ammo loaded to even higher pressure/velocity levels. There are standars for +P but not for +P+.
+P is commonly loaded in: .38 special, 9mm, .38 super, and .45 ACP. +P+ is most commonly found in LE ammo in 9mm and .38special.