Biomes

Environments (or Biomes) are different types of landscapes located around a Terraria world. The most common biomes are Forests and Deserts, located on the surface. Going farther left or right will eventually lead to the Corruption and the Jungle. On each end of the map 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 the Wall of Flesh in The Underworld, new points of corruption will form, as well as a new environment (The Hallow).

Each environment has its own unique items as well as perils (including underground versions of themed biomes).

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, and be used to grow Trees. Vines can grow from the bottom of grass covered dirt blocks if they have a space beneath them. Despite 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 they only spawn weaker varieties of slimes during the day.

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. Because sand is affected by gravity, it's very easy to mine by simply digging a few feet down and working in a horizontal line. Don't stand next to the sand column while doing this, as falling sand does damage when it lands on top of a player. Cactus also grows here.

Corrupted Deserts & Hallowed Deserts
When the world enters Hardmode, several deserts of the world will become either corrupted or hallowed. These two sub-biomes have their own backgrounds, and spawn unique versions of the Mummy (the Light Mummy and Dark Mummy, corresponding to the biome) which drop components needed to craft the Dao of Pow. Sand will be replaced with Ebonsand and Pearlsand, much like Stone is replaced in other themed biomes.

The Underground
Below the 0ft depth level 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, other than causing Goldfish to become Corrupt Goldfish. 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 can be an annoying opponent for characters with low-level equipment, so make sure to have plenty of room to maneuver when fighting these.

The Cavern
As the biggest biome in Terraria, The Cavern is massive - on Large maps, it is easily 50% of the entire world. It encompasses the Underground Jungle and the bottom part of the Dungeon. It's also home to a harder set of monsters (including Skeletons, which drop the much-needed Hook). The treasure here is similar to the Underground, but with occasional patches of Glowing Mushrooms.

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 comparable to Hell. But the risk is compensated by reward - within the Ash and lava wells are deposits of Hellstone, the most powerful crafting material in the game (before Hardmode). But Hellstone is also dangerous to touch, so it would be wise to you take an Obsidian Skull or a similar item to prevent damage. As of version 1.0.6, any Hellstone in the Underworld layer will now create lava when mined. There are also towers of burning-hot Hellstone Brick or the relatively safe Obsidian Brick. You can also find Hellforges down here, which is an upgrade to the Furnace. A wide variety of dangerous monsters can spawn down here: 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 powerful flying monsters that can deal a lot of damage unless you're properly equipped. It is highly recommended that you do not enter The Underworld unprepared; Death does happen even to the most seasoned players.

Jungle
The Jungle is one of the surface biomes, 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.

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 thorns. The monsters that spawn here (Hornets and Man Eaters) can do some impressive damage, with the Man Eaters dealing 60 damage to unprotected players. The treasure here is considered high tier.

The Corruption
Every world has a section of Corruption. Small worlds have 1 or 2 sections (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 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. (In Hardmode, the spreading Corruption will uproot sunflowers. Try placing Ash or Clay instead.) The purple stone found in the corruption is called Ebonstone, and can only be mined by Dynamite or any pickaxe/drill more powerful than a Gold Pickaxe. 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).

Underground Corruption
The Underground Corruption is created in the Cavern layer upon defeating the Wall of Flesh. It shares the same feel as the overworld Corruption, but has different enemies, a different soundtrack (a mix of the regular Underground and Corruption soundtrack), and it's the only place to find Souls of Night and Cursed Flames. All enemies from the overworld version are present, as well as World Feeders, Clingers, and Cursed Hammers.

The Hallow
The Hallow is the "anti-corruption" biome, which is automatically created once the player activates hardmode by defeating the Wall of Flesh. It can be created with Hallowed Seeds and will stop the Corruption from spreading, but it also takes over adjacent biomes. The biome has light blue grass and colored trees. It also has its own underground biome. 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 occurs when The Hallow spreads in the Cavern; this is easily noticeable because it's the only place where Crystal Shards spawn on blocks. Illuminant Slimes, Illuminant Bats, Chaos Elementals, and Enchanted Swords can be found here. It has a different soundtrack from the overworld Hallow (the same one used in the 1.1 trailer), and is the only place to get Souls of Light.

Dungeons
Along with the Underground Jungle, the Dungeon is considered one of the higher-end biomes. The dungeon is a very large Pink, Blue, or Green brick building with an Old Man near the door. Spending too much time in the Dungeon before Skeletron is defeated results in almost-inevitable death. The dungeon itself runs very deep with many passages leading around and in many directions. Spikes also cover floors and walls in many parts of the dungeon, and do considerable damage to players (but not monsters). The items found here are rare and valuable, 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 - while you hold one, monster spawn rates are increased. The books located around the dungeon also may house the Water Bolt spell. When entering 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).

Meteorites
This mid-tier biome is special: whenever a Shadow Orb is smashed, there is a chance that within the next seven days the game will spawn a meteor and display the message "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, an Obsidian Shield, or an Obsidian Horseshoe 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 are more than 50 blocks of Meteorite left in the area, Meteor Heads will spawn. These monsters can fly through solid objects 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. Despite the danger, Meteorite gear is a great step up for the newer player.

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 a 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 monsters that spawn here will be the same as the monsters as from the Forest biome, as well as Harpies (and Wyvern on hardmode). Although rare, Meteors can hit a floating island.

Oceans
At each end of every map there will be a deep body of water. This water is not infinite, and can be drained like any other body of water. Oceans will sometimes spawn with underwater chests, usually quite a distance away from shore. The items 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.

Snow Biome
Snow Biomes are Christmas-themed biomes, in which any kind of dirt is replaced by Snow Blocks. It doesn't feature a special soundtrack or background, but it's the only biome with a particle effect (falling snow flakes). It is resistant to both the Hallow and Corruption biomes. This biome will always be included in a world created during the holiday season. Worlds created during the rest of the year have a 1/3 chance to be generated with a snow biome.