N is the leader of Team Plasma and the only known son of Ghetsis. His goal throughout Pokémon Black and White is to free Pokémon from humans by attempting to create an entirely new world for them to inhabit. His full name stands for "Natural Harmonia Gropius" (or in Japanese "ナチュラル・ハルモニア・グロピウス" that means Natural Harmonia Gropius).
In Pokémon Black and White N is the leader of Team Plasma (often refered by members to as Lord or King) who wishes to separate Pokémon from the human race. Through your journey you will encounter N many times and often battle him. His team will change each time he battles you, capturing the Pokémon of the area he battles you in, then later releasing them.