If you haven't heard about it, there are two simple rules:
You can only catch the first Pokemon you see in an area.
If a Pokemon faints it is considered dead and you must release it.
In reference to rule 1:
If you cannot catch a Pokemon yet, this rule still applies. It's by sheer luck you don't see anything in the first area of the game. Yes it counts on Ruby, Sapphire and Emerald as well with the Poochyena fight. But you can alter this rule, but it wont be a true Nuzlocke in my view.
In reference to rule 2:
It's possible for first time Nuzlockers to add a "mercy" rule. Such as, "Your Pokemon can faint 3 times before it is permanently lost." Or however many times you wish. It's recommended to stay between 1-5. Or you can set a level cap. Like once it hits level 20 or 30 it cannot faint anymore or else it's considered dead. Your choice, but I'd stick to the "no exceptions" rule.
Basically it's game over if you lose all of your Pokemon.
Pretty basic rules. You can add whatever else you'd like. They're self imposed so keeping to the rules is your own responsibility. So don't make it too difficult on yourself. Or too easy.
This is mostly a topic to keep up to date on my own Nuzlocke challenge, but if you want to start one you're more than welcome to make your own post and keep your progress as well~ I just didn't see a Nuzlocke topic anywhere. Especially considering there hasn't been a new post since...2009? O_O'' I kinda wanna revive the forums a little bit. |D *will most likely end in failure*
Current Nuzlocke:
Pokemon Blue (Original copy - playing on Game Boy Color)
Name: Nuzlock (couldn't fit the e. ^^;)
Starter:
Supporters (In order of capture):
July 13th
Started
Lost Charmander in the first battle with Gary (Mercy rule added.)
Caught Rattata
Lost Rattata
Caught Pidgey
Lost Charmander
Caught Kakuna
Caught Caterpie
Evolved Kakuna into Beedrill (still only knows Harden...>_<)
Evolved Caterpie into Metapod
Currently: Training for the Pewter Gym.