Slimes

Slimes are the most common enemy type. They come in multiple sizes and colors, each with different health and damage. All slimes follow basic Slime AI and attack by hopping onto the player (with the exception of the Gastropod). When a slime is preparing to hop, it bobs up and down rapidly. Slimes float in water. Surface slimes will not purposely hurt you unless you attack them, or it turns night.

Slimes spawned on or near the surface are usually neutral and do not chase the player during the day, unless provoked. They become hostile at night. Slimes spawned underground are always hostile. Green, Blue, Red, Purple and Yellow Slimes usually spawn on or near the surface. Mother Slime, Baby Slime, and Black Slime can only be found underground. Pinky may appear anywhere, although rarely.

Most types drop 1-2 gel, but there are numerous exceptions. Gel is a vital ingredient, as it crafts Torches, which are of use throughout all stages of the game. As of 1.3, regular Slimes, except Dungeon Slime, will sometimes drop a potion or other bonus item on death, and this item will be shown inside the slime's body.
 * Corrupt Slimes and Crimslimes drop 2-4 gel
 * Lava Slimes and Dungeon Slimes drop no gel
 * The King Slime drops no gel, but the slimes it spawns can produce a large amount of gel
 * Pinkys drop 25-50 Pink Gel
 * Rainbow Slimes drop 1-20 gel
 * Gastropods drop 5-10 gel
 * Umbrella Slimes drop 2-5 gel

Bonus drops
On the PC version, naturally spawned slimes that have the same shape as a Blue Slime have a 1/20 chance of carrying a bonus item.

Slimes spawned from statues can never carry a bonus drop, and Dungeon Slimes will always have a Golden Key as a bonus drop.

1When in single player mode, this item will be replaced by Recall Potion.

Trivia

 * Slimes are hinted to be made of an acidic digestive substance by the Nurse in her dialogue: "You look half digested. Have you been chasing slimes again?"

History
. ..

Slime Slime Slime Слизень