Biomes

Environments (or Biomes) are different types of landscapes located around a Terraria world. The most common ones are Forests and Deserts, located on the surface. Going farther left or right will eventually lead to the Corruption and the Jungle. On each side there are large bodies of water called Oceans. Underground environments are also present in the forms of the Underground, the Cavern, and The Underworld. The sky itself has its own environment called Floating Islands. After defeating Wall of Flesh in The Underworld, New points of corruption may form, as well as a new environment, The Hallow.

Each environment has its own unique items as well as perils.

Forests
Forests are the most common environment that the player will come across. Green grass grows across the dirt, which can spawn mushrooms and Daybloom. Vines can grow from the bottom of grass covered dirt blocks if they have a space beneath them. Contrary to its name, forests are still considered the same Biome even without the presence of trees. However, trees can only grow on grass covered dirt. Forests are considered lower end environments because of the spawning of the weaker slimes during the day time.

Deserts
Deserts are large expanses of Sand. They are quite common between Forests, and spawn Vultures and Antlions, which new players should be wary of. Due to the way sand works when dropped in gaps; it's very easy to mine by simply digging a few feet down and working in a horizontal line. However, take care that you stand a bit away from the sand when doing this as falling sand does damage when it lands on top of a player. Cacti also grows here, which used to deal damage although is harmless as of 1.1.

Oceans
At each end of every map there will be a deep body of water. However, this water is not infinite, and can be drained like any other body of water. Occasionally maps will spawn with underwater chests, usually quite a distance away from shore. The items contained inside will often be water related, like Flippers and Breathing Reeds. The ocean is a relatively dangerous place for the newer player as it houses monsters like the Shark and the Jellyfish.

Jungle
The Jungle is one of the surface biomes, similar to the Underground Jungle. It is composed primarily of Grass, Vines, Mud and Jungle Trees. The sky turns a vibrant green within the jungle. The jungle contains Piranha, Jungle Bats, and other creatures. The Jungle Trees can not be grown intentionally, but instead randomly grow without the use of acorns.

The Underground
After reaching a depth of 0ft and going below you will be in The Underground. The music will change when you pass this point, and stronger monsters will spawn. This layer is considered completely separate from the surface, so if a Blood Moon rises it will not affect you. Compared to the rest, The Underground is the second biggest biome. The treasure found here is mid level, and very valuable to players who are ready for more adventure. The player may also encounter their first Worm: The Giant Worm. This is a very tough opponent to fight without preparation, so make sure to have plenty of room to maneuver when fighting these.

The Cavern
As the biggest and deepest biome in Terraria, The Cavern is massive. On Large maps it is easily 50% of the entire world. It also encompasses the Underground Jungle and the bottom part of the Dungeon. It's also home to a harder set of monsters, including the Skeleton, which drop the much needed Hook to make Grappling Hooks. The treasure here is similar to the Underground, however you will find patches of Glowing Mushrooms, which are used to make the most powerful healing consumables in the game.

Underground Jungle
The Underground Jungle is one of the higher tier biomes in the game, and is found at Cavern depth under the surface Jungle. The jungle will have vegetation in the form of vines, flowers, glowing seeds, and dangerous spiky vines. The monsters that spawn here (Hornets and Man Eaters) can do some impressive damage, with the Man Eaters dealing 60 damage to unprotected players which is the most powerful single strike a monster can do. The treasure here is considered high tier.

The Underworld
Being by far one of the hardest levels to reach and,more importantly, to survive in; The Underworld is a dangerous lava-filled area akin to hell or hades. But trips here can be quite a fruitful endeavor seeing as between the Lava Wells are deposits of Ash and Hellstone, the latter being the most powerful crafting material in the game. But Hellstone is also dangerous to the touch, so it would be wise during any trips down to The underworld that you take the Obsidian Skull or similar items to prevent damage. Hellstone was made even more deadly in 1.0.6, as it will now create lava when mined. There are also Hellstone Brick buildings that have their dangerous hellstone properties. You can also find Hellforges down here, which is an upgrade to the Furnace. A wide diversity of monsters can spawn down here as well such as the: Fire Imps, Bone Serpents, Lava Slimes, Demons, and Hellbats. The Fire Imps use the Caster AI and will chase the player. Bone Serpents are more powerful Giant Worms that are much larger and harder to kill. Lava Slimes swim straight over the lava towards you. Hellbats and Demons are also powerful flying monsters who will deal a lot of damage if improperly equipped. It is highly recommended that you do not enter The Underworld unprepared: Death does happen even to the most seasoned players.

The Corruption
Each and every world has a corruption. Small worlds have 1 or 2 corruptions (usually with one in either direction from the spawn point), Medium worlds have around 2 or 3, and Large worlds can have up to 4. The monsters that spawn in the corruption are both powerful and spawn regardless of the time of day. The purple grass of the corruption can be removed with Purification Powder or the quick swing of a pickaxe. The spreading of the corruption can be prevented by either placing any block other than dirt at its edge, or by planting a Sunflower, which can be found randomly in the world or bought from a Dryad. The purple stone found in the corruption is called Ebonstone, and can only be mined by a Nightmare Pickaxe and Molten Pickaxe or Dynamite. A newer player should not try to cross The Corruption without a set of decent gear: as well as either a good grasp on block placement for pits or an item that allows vertical travel (like a Grappling Hook).

The Hallow
The Hallow is a "anti-corruption" biome. It can be created with Hallowed Seeds and will stop the Corruption from spreading, but it will automatically spawn once the player activates hardmode by defeating the Wall of Flesh. The biome has light blue grass and colored trees. It also has its own underground biome. Biome name was taken from bandcamp page. The Dryad will tell the player the percentage of Hallow and Corruption in the world, after the player has defeated the Wall of Flesh boss in the Underworld. Unicorn and Pixie mobs will spawn here.

Underground Hallow
The Underground Hallow is the cavern of The Hallow, the Illuminant Slime can be found in this cavern.

Meteorites
This mid tier biome is special: Whenever a Shadow Orb is smashed; there is a chance in 1-7 days that the game will spawn a meteor and informs the player "A meteorite has landed!". The meteor will crash land and turn a large portion of the surrounding tiles into Meteorite. Meteorite will burn the player if they do not have an Obsidian Skull equipped. Meteorite also requires the player to have a Gold Pickaxe or better, as lower level pickaxes cannot break it into ore. Dynamite can also be used to great effect. While there is more than 50 blocks of Meteorite left in the area, Meteor Heads will spawn. These monsters can fly through all soils and ore, and will ram the player for a respectable amount of damage. One of the best ways to deal with them is to use a vilethorn, as one shot can kill a whole cluster of meteor heads. It's worth getting however, as Meteorite gear is a great step up for the newer player.

Dungeons
Along with the Underground Jungle, the Dungeon is considered one of the higher end biomes. The dungeon can be located by finding a Pink, Blue, or Green brick building, with an Old Man near the door. Until Skeletron is defeated (which the old man turns into at night); entering the dungeon is dangerous at best or will outright kill the character. The dungeon itself runs very deep with many passages leading around and in many directions. Spikes also cover the floors in most part of the dungeon, and do considerable damage to players (but not monsters). The items found here are rare and unique, and some aren't found anywhere else. The monsters that spawn here are powerful and come in great numbers. There are also Water Candles around the dungeon. If picked up, you will get more monster spawns. The books locate around the dungeon also may house the Water Bolt spell. If coming into the dungeon for the first time make sure you have a full stack of potions and plenty of Inventory space. It also helps to have some kind of vertical transport (Rocket Boots or a Grappling Hook).

Floating Islands
The Floating Islands are some of the smallest biomes that the player will come across. Due to their location in the sky they are also one of the harder biomes to reach. Finding them is not an easy task, but can be accomplished by using a Gun with Meteor Shot or the Water Bolt spell, both of which bounce off one object. Once found, simply build a tower or use the Gravitation Potion to float up to it. The items on Floating Islands are very useful, and will serve most players well even in the end game. The monster that spawn here will be the same as the monsters as from the Forest biome, as well as Harpies. Although rare, Meteors can hit a floating island.