I believe this is referring to the zero chunk, or chunk 0, 0.

Note that since the Halloween update, the zero chunk can't be exploited as the most efficient spawn location, so you're better off just building a mob tower anywhere, rather than trying to find your zero chunk.

Minecraft Forum user Ruzhyo2000 posted a little tutorial on how to find your zero chunk.

1. Download cartographer.

2. Make a map of your world using cartographer.

3. Make a backup save of your world.

4. Go into your world folder (the one that is still in the .minecraft/saves folder) and go into folder 0. From there, go into folder 0 again. The chunk file in this folder is for chunk 0,0. Delete it.

5. Now make a map of your world using cartographer again. The chunk that is missing is chunk 0,0.

6. If you have MCEdit, use it to go into your world folder that has chunk 0,0 deleted. Using this, you can find the exact location of the chunk's 16x16 bounding box, down to the exact coordinates, by visiting this chunk location. You cannot do this in game because the chunk will just regenerate.

7. You have two options. If the chunk was in the middle of nowhere (such as the ocean) you can outline the bounds of this chunk with cobblestone or something using MC edit and then go back into the world. When you get close enough to chunk 0,0, it will just regenerate. However, if the chunk was over structures you placed or a mine that you wish to keep, just note where the chunk is exactly and replace your changed save with your back up.

8. Profit!

Now, the second part of your question sounds like it's referring to a mob spawning tower. Different from the mob spawner block you find in dungeons, a mob spawning tower is basically a really dark set of rooms that, using currents of water, funnels mobs that spawn in it into into a single stream, kills them, and brings the items somewhere. See this answer on Gaming.SE for more info.

Technically, placing torches will decrease your overall mob spawns; the game does pick a single random point within the eligible chunks to spawn a monster, and if that point is not suitable, the monster doesn't spawn. Therefore filling other caves with torches will reduce the number of spawn-friendly points, reducing your overall monster count. This does technically help your mob grinder because you don't have mobs lurking in neighboring caves counting against the mob cap (and ultimately taking it all up, preventing anything from spawning in your grinder).

The best solution would probably be to use a vast water network to drag all the mobs from all nearby caves into your grinder, so you maximize spawn area, but bring any mobs from all the viable spawn points into a single area.