Slime Monsters

Slimes are a family of monsters, identified as slightly transparent blobs that slowly hop toward the player in an attempt to attack. It is the most common enemy in Terraria, able to spawn at any time of day and in nearly every biome. Slimes come in a multitude of colors, including blue, purple, green, red, yellow, and black. Upon being slain, Slimes drop Coins and Gel, which can be used to create Torches, Sticky Bombs and Potions. Slimes are the biggest threat to new players, due to them bombarding the player almost right away and wasting valuable time if they kill the player. Slimes are not aggressive during the day and happily jump across the level minding their own business. They will still hurt if touched. Attacking them will cause them to become aggressive and attempt to jump at the player. They will also become aggressive when night falls. Slimes found in caves are always aggressive.

In earlier versions of the game, the Slime spawn rate was stated by Blue to be far beyond reasonable, leading the developers to be subtle. Blue dubbed the neverending onslaught as "Slimepocalypse".

Strategy
Slimes' movement and tactics are very simple, being limited to hopping in one direction with short pauses between each leap. The simplest way to avoid slimes is to block their path. Placing Wood Platforms at a natural choke point is a reasonable alternative, as slimes cannot fall through them.

Special Slimes
A large black slime found in deeper caves. The Mother slime is 3 blocks wide and 2 blocks tall, making manuvering in tight areas more difficult. Upon death, it splits into two or three smaller and weaker 'Baby Slimes'. Using a weapon with a large area of effect and high damage, such as a flail, large sword, or explosives, is an efficient way to dispatch the Mother Slime and its "offspring" at the same time.
 * Mother Slime

A tiny pink slime found on or near the surface which has 150 health. On death, it drops 60+ Silver Coins.
 * Pinky

Habitat-Based Differences
Green & Blue Slimes spawn regularly on the surface during the day. Purple Slimes also spawn on the surface, although less frequently. Pinky is the last surface-dwelling Slime, and has the rarest spawn rate. During the day, all of these Slimes are happy to ignore you, so long as you don't attack them first. However, if they are spawned already when night arrives, they will become hostile to you. None of these slimes spawns above ground during the night.

Blue & Red & Yellow Slimes spawn in the Dirt layer fairly regularly, with Pinky also spawning rarely as well. Red & Yellow slimes are always hostile, while Blue Slimes & Pinky are sometimes willing to ignore you until attacked.

Black & Mother Slimes spawn regularly in the Stone layer, as well as rarely in the outer Underground Jungle biome (becoming more rare the deeper into the biome you travel). As with Red & Yellow Slimes, Black & Mother Slimes are always hostile on spawn. Baby Slimes, which spawn as a result of killing Mother Slimes and look like Black Slimes, are also always hostile.