Slimes are little jelly-like blobs that can only be found in a map section with only one entrance, usually a very long way from the entrance to that level. When killed, they always drop a Planar Gate.
There is a Lord Slime, but the conditions to cause it to appear are unknown. It has no armor. It will spawn a large number of regular Slimes when its health is 1/4 or below. The spawned Slimes do not have armor.
Slimes do not attack. Instead, they run away from the character, trying to stall out. Usually the only way to damage it long enough to kill it is to trap it against a wall. If it isn't killed within a certain period of time, it simply disappears. It will also disappear if it manages to reach the exit of the room.
Slimes that are roughly the same level as the character will be difficult to defeat without high level skills or equipment, because the armor is extremely high, and same-level characters simply won't be able to break the armor fast enough. It's recommended to re-tread easier dungeon levels searching for slimes, as the Planar Gate functions the same no matter the level of slime killed.