Slimes are rare hostile mobs. Found in swamps, or underground caves, they will commonly spawn in the world type Superflat, this is due to the default Superflat world being only three blocks above the bedrock layer. As of 1.5 slimes spawn rate is determined by the movement of the moon. Slimes have four different sizes, and are one of two that have this quality, the other being magma cubes. Their sizes are large, medium, small and tiny, the tiny size being harmless to the player.
When killed, one big slime will split into two medium slimes, which will split in to multiple Tiny Slimes. Only tiny Slimes drop 0-2 slimeballs, items that are used for crafting Sticky Pistons. Slimes will drop 1-4 Experience, depending on the size of the slime. Slimes spawn in layer 40 and lower, in any light, and in any size rooms. However, only certain chunks will spawn slimes. You can find these chunks by entering the seed for your map in this tool here. Hacked Slime sizes can be added onto the game. They aren't able to swim (which means that they drown in water) or jump out of water.