Nuzlocke is a way to play pokemon. Basically it's rules like. If a pokemon is KOed, you have to release it. No pokemon center. You can only catch the first pokemon you see on a route, etc.
Wait, I'm pretty sure the Pokemon Center thing is one of the optional rules, right?
I'm just using the core rules this time, which are:
-Any Pokemon that faints is considered dead, and must be released.
-The player may only catch the first Pokemon encountered in each area, and none else. If the first Pokémon encountered faints or flees, there are no second chances.
But you can add extra rules if you want, such as the Pokemon Center thing or perhaps no Potions or not letting your Pokemon evolve.