what the game counts as a biome
Terraria does not keep a map of named regions. It counts tiles around you, constantly, and decides where you are from the tally. The sample area reaches 84 tiles to your left and right, 62 up and 61 down from your center, which is the same reach a Campfire or a Bast Statue has. Cross a threshold and the music, background, water color, enemy list and fishing table all flip at once. Walk back out and they flip back.
Thresholds vary a lot. The Hallow is the cheapest at 125 blocks. The Jungle wants 140. The Corruption and Crimson need 300 each. Snow needs roughly 1,500 snow or ice tiles. Backgrounds usually lag one tile behind the functional switch, so an area can already spawn the right enemies while still showing the wrong wall. Tiles you have hit with an actuator still count toward the total, which is how compact biome farms fit inside a small room.
the world is a stack of layers
Space caps the world. Gravity is weaker there, the stars stay out in daylight, and it holds the Floating Islands with their Skyware Chest loot. Below that, the Surface and the Underground share the same biomes, split at the 0 feet mark, and the true subterranean versions do not begin until the Cavern layer. That is where the Ice biome, the Underground Desert, the Underground Jungle and the glowing mushroom patches live. The Underworld runs along the very bottom, wall to wall.
The practical consequence is that a hole straight down passes through three separate enemy pools before it reaches lava. A shaft dug in the snow gives you snow enemies, then ice biome enemies, then Underworld ones, and the gear that carried you through the first stretch is usually the wrong answer for the last.
the layout is not random
Every world mirrors itself around the center. The Snow biome and the Dungeon always sit on the same half, and the Jungle and the Desert always sit on the other. Oceans occupy both edges. The Aether, with its shimmer pool, generates near an outer edge on the Jungle side. One landmark therefore tells you where two others are. Spot penguins and you know which way the Dungeon lies, and that the Jungle is a long walk in the opposite direction.
the three biomes that move
Corruption, Crimson and the Hallow are the only biomes that grow on their own. Before Hardmode they creep through grass and thorny bushes alone, one tile at a time, slowly enough to ignore. Killing the Wall of Flesh changes that: the Hallow appears as a diagonal band cut across the whole world, and all three start converting stone, sand, ice, dirt and vines within three tiles of any infected tile. Beating Plantera for the first time halves the rate again, permanently.
biomes you build yourself
Several biomes exist only because you made them. Three NPCs living close together form a Town, which shuts off ordinary enemy spawns in Classic and cuts them hard in Expert and Master. Seven tombstones in one spot form a Graveyard, which produces ecto mist and lets night enemies spawn in daylight, and it never generates naturally. Beyond that you can seed the Hallow, either evil, mushroom, jungle, desert or snow anywhere between the Underworld and Space.
One trap when building a Hallow room: the Hallow and evil counters cancel each other out. Each Hallowed tile adds one to the Hallow total and subtracts one from both evil totals, and every Corrupt or Crimson tile does the reverse. A room sitting at exactly 125 drops back to neutral the moment one infected block creeps in. Build with slack.
overlaps, hybrids and one room prizes
Biomes stack. A desert caught in the Hardmode conversion becomes a Corrupt, Crimson or Hallowed desert with its own Mummy and Sand Shark variants. Ice does the same and starts producing Pigrons. Floating Islands convert too, and a few Angler quests only accept fish caught in exactly those hybrids, which is why some quest fish look impossible until you realize you are meant to build the biome.
The rest of the world is dotted with set pieces that are not really places to live: spider nests holding the only Web Slinger chests and the Stylist tied up somewhere inside, granite and marble caves with their own enemies, gemstone caves, glowing moss caverns, pyramids, and shrines built around a background sword that has a small chance of yielding Terragrim rather than the ordinary Enchanted Sword. You visit, take the one thing worth taking, and move on.