The Slime is a hostile rare large green block shaped mob. It can be found in caverns at very low depths. But as of 1.4.7 they can also spawn in the swamp biome. Their rate is also determined by the phase of the moon. They spawn as either huge, large, small or tiny slimes. The huge slime was actually removed from the game but can be spawned in with singleplayercommands along with the giant. When a large slime is killed they split into four small slimes, and when a small slime is killed they split in turn into four tiny slimes. When a tiny slime is killed it has a chance of dropping up to two Slimeballs. Slimeballs won't drop if the slime is killed by a wolf or lava. Slimes are only hostile to players.
Larger slimes will deal more damage, and the smallest slime won't deal any damage when it attacks. For this reason, tiny slimes are sometimes kept as pets. Tiny slimes can push the player.
Slimes will hop toward the closest player within 16 blocks, whether they can or can't see the player. Slimes can be harmed by burning, fall damage, drowning, suffocation, lava and/or weapons. Keep in mind slimes can beat the current of water, so don't use water to push them back. Using a bow is always a safe method, but their hopping may make them difficult to hit. Although they do hop in a straight line toward you. A sword can be used, but be careful as they'll split until a tiny slime. You're best attempting to fight them one by one if you can.