Terraria is a 2D sandbox game with gameplay that revolves around exploration, building, crafting, combat, and mining. The game has a 3D sprite tile-based graphical style reminiscent of the 16-bit sprites found on the SNES. The game is noted for its classic exploration-adventure style of gameplay, similar to games such as the Metroid series and Minecraft.
The game starts in a procedurally generated world. The player starts out with three basic tools: a pickaxe for digging, a sword for combat, and an axe for woodcutting. Many resources, notably ores, can be found while exploring caves. The player begins with a limited amount of health and mana points, which can both be increased by finding certain items. Some resources, and most items, may only be found in certain areas of the map, stored in common and rare containers, or dropped by certain enemies. The player uses resources to craft new items and equipment at an appropriate crafting station for that recipe. For example, tables or other items can be crafted at a work bench, bars can be smelted from ore at a furnace, and armor can be crafted at an anvil. Several advanced items in Terraria require multiple crafting operations where the product of one recipe is used as an ingredient for another.
The player can encounter many different enemies in Terraria, such as simple slime blobs, zombies, skeletons, flying eyes, and various biome-specific enemies. The occurrence of certain enemies depends on several factors including time, location, random events, and player interactions. The player can fight against enemies with swords, bows, guns, magic spells, and summoned attackers. The player may also battle bosses with a number of different combat mechanics that can drop rare items and large amounts of in-game currency. All bosses can be summoned by using certain items or randomly when certain criteria are met. Some of the bosses can spawn naturally in certain places and times. The defeat of these bosses is directly tied to in-game progression.
By completing specific goals, such as defeating a boss or obtaining a certain item, players can attract non-player characters to occupy structures or rooms they have built, such as a merchant, nurse, or wizard. Some NPCs can be acquired by finding them throughout the world and will then reside in the player's house like the wizard or the mechanic. Players may then buy or sell items and certain services from NPCs with coins found in the world. The game includes a currency system in the form of coins that can be primarily used to complete transactions with NPCs.
The game recognizes many different biomes and areas, defined by the blocks that exist in the vicinity, each home to a unique set of enemies. Some biomes, such as the Corruption or Crimson biomes, will naturally expand by slowly overtaking and converting blocks in adjacent biomes. If the player journeys to the Underworld biome and summons and defeats a boss called the Wall of Flesh, the game enters "hardmode", which adds many new and much stronger enemies throughout the world, as well as new NPCs, bosses, ores, items, and a new biome called The Hallow.
The game also features an "expert" difficulty, which can be enabled when creating a world. Expert mode increases the difficulty of the game by increasing the attack and health of monsters and bosses, further increasing the strength of weaker enemies after hardmode, giving some bosses new attack patterns, decreasing the effectiveness of life regeneration, and other tweaks. To deal with the higher attack damages of expert mode the defense statistic is more effective than it is in the normal difficulty. In addition to the higher difficulty level, expert mode increases the chance for enemies to drop rare items, and adds new items that can only be obtained by defeating the bosses on expert mode.