Forest

Forest is what the surface is when it is not something else. Grass, trees, ponds of every size, and rolling hills that occasionally open straight into a cave mouth. It covers most of the world above ground and it is almost always where you spawn.

It is the easiest biome in the game and that is deliberate. The enemies stop mattering the moment you have any equipment at all, natural spawn rates are low, and the first three materials you need are within a minute of where you land: Wood, Stone and Clay. Surface chests are scattered across it and their loot table is genuinely useful rather than filler.

There is also a chance of low tier ore breaking the surface here. Copper or tin, iron or lead, and sometimes silver or tungsten can sit in the open alongside pockets of clay and stone, which occasionally saves you the first trip underground entirely. Sakura and yellow willow trees replace an ordinary forest tree about one time in twenty at world generation, so a stand of them is normal rather than a sign of anything.

The forest changes once you get past roughly the outer sixth of the map on either side. Goblin Scouts start appearing out there, uncommon but worth hunting, since Tattered Cloth is what you need to build the standard that starts a goblin invasion on your schedule rather than the game's. King Slime also has a small chance of spawning on its own in that zone, with no warning, which is how an under-equipped character exploring the edge of the map gets flattened. Enchanted Sword Shrines only generate in the outer thirds, so if you are hunting an Enchanted Sword, stop searching the middle of the map.

Living Trees are the oversized ones with hollow interiors, and some of them run a shaft down to a small treasure room. Mountain Caves are the other structure to know: a tall dirt mountain with a zigzag passage cut through it that drops you underground with no digging at all, usually with pots and wooden chests along the way. Early on it is the cleanest route down that exists.

Fallen Logs lie on the ground and give up wood to an axe. Fairies spawn around them at night and lead you to something nearby worth having. Chopping every log in the world permanently ends surface fairy spawns, so leave at least one standing. Shaking trees still produces fairies afterward, but that is a worse deal than simply not breaking the last log.

Nothing about forest resists corruption, crimson or hallow, and all three will overwrite it. Hardmode also adds a handful of new night spawns here, which is a mild change rather than a real one. For small patches in pre-hardmode, Purification Powder from the Dryad is enough. Once hardmode starts the area involved makes powder pointless and you want a Clentaminator loaded with Green Solution from the Steampunker.

Forest is the most popular biome in the game for NPCs by a clear margin. Only two dislike it and four actively like it, and only the hallow comes anywhere near that. That is why the standard advice to put your main town in the forest holds up under scrutiny: more residents pay you back with lower prices here than anywhere else, and the Forest Pylon is the one you will use most.