That's right, swarms. Here's a link:
http://www.d20srd.org/srd/monsters/swarm.htm
It says occupy a "10 ft space" does that mean a 10ftx10ft square? Or a Line ten feet long? The square would make more sense. Any other tips, tricks, advice, or helpful hints about swarms in general would be useful.
Aslak is correct. If you read down through the third paragraph of the Swarm entry it says "Swarms of nonflying creatures include many more creatures than could normally fit in a 10-foot square based on their normal space... ."
Are you playing 3.5 or Pathfinder? The PRD clarifies things a bit more, with non-flying creatures taking up a 10ft square, while flying creatures take up a 10ft cube.