HP stands for Hit Points. This is the amount of damage a Pokemon can take before fainting. It can also stand for the move Hidden Power.
PP stands for Power Points. This is the number of times you can use a move before you can't use it again until replenishing those Power Points.
EVs stand for Effort values. These increase stat amounts. At level 100, for each 4 EVs you have in a stat, that stat increases by 1. See http://pokemondb.net/ev
OU is a tier standing for Overused. It's the standard tier of battling. A tier is a guideline that separates Pokemon into different levels based on how good they are. See http://pokemondb.net/pokebase/meta/5256/official-tiers-tier-explanation-and-bans-for-the-pokemondb
BS stands for ****, and has nothing to do with Pokemon. Kidding, Battle Subway, a section of the site or also the Battle Tower of Gen V, where you face opponents to gain points to purchase items. See for the site one http://pokemondb.net/pokebase/rmt/
IV stands for Individual Value. Like EVs, IVs increase stats. However, they cannot be increased and are as they are when you first obtain the Pokemon. It also determines Hidden Power type and power. See http://pokemondb.net/mechanics/hidden