Sorry, when I say mob grinder I mean mob farm. A mob grinder is the killing mechanism that is part of a mob farm, but I'm really talking about the entire farm itself. A mob farm is basically a structure that is designed to create optimal conditions inside for monsters to spawn. The structure then forces them into an area where they are killed, thus dropping their loot. If built well, they can constantly spawn very large numbers of mobs, even during the day, producing a constant flow of mob items, such as gunpowder. To increase the spawn concentration, it's best to eliminate as much of the surrounding terrain as possible from possible spawning spots. As such, they're best built in areas that have been completely lit above and below ground (difficult to accomplish), or out in the middle of the ocean. The one I built didn't work well because it was underground in my lair. Because there was a lot of unlit area all over the place nearby, particularly underground, the spawn rate of the system was really, really low. I actually started preparing an island for a better grinder, but never actually started construction. The island is at x=970 z=-380.

http://www.minecraftwiki.net/wiki/Tutorials/Mob_Farm

There are tons of youtube tutorials on various configurations.