Coins

Coins are Terraria's unit of currency. They are primarily used for NPC transactions, though other players may also accept them as payment in multiplayer.

Types of coins
There are four types of Coins in Terraria: Copper, Silver, Gold and Platinum. Each is worth 100x the coin of the previous value. One Platinum Coin is worth one million Copper Coins.

Obtaining Coins
Coins can be acquired by killing monsters, breaking pots, or trading with NPCs (or other players). Even though the Coins are "Copper", "Silver" or "Gold", it is impossible to craft them from Ore, Bars, or any other item in general (apart from a different rank of Coins)

Coin harvest techniques
An easy way to acquire Coins is to harvest Sand Blocks, craft the sand into Glass then Bottles, and then sell those to an NPC for 4 copper per tile of sand. Crafting Ale via Mugs and a Keg is even more profitable, as it turns 2 sand into 20 copper for an exchange of 10 copper per unit of sand (or 25 silver per full stack).

If you have advanced through the game to the point of acquiring a Bucket and Nightmare Pickaxe, you can create an Obsidian Generator to mass-produce Obsidian Skulls. These sell for 54 silver each, so selling just two of them gives you over 1 Gold Coin. With this technique, it is possible to acquire the Minishark, with a price of 35 gold, in half an hour.

Obtaining Coins by battling Meteor Heads is also an easy option, given their constant spawn rate, but it requires 50 Meteorite and preferably something good to kill Meteor Heads with. (Arenas guide on farming Meteor heads)

You can also farm the Eye of Cthulhu for Demonite Ore if you have good enough weapons and armor. A single Eye will yield an average of about 50 Demonite Ore each time, selling for 8 silver each (about 4 gold in total). You can also farm the Eater of Worlds, which will net about 10 gold if everything is collected (although it is arguably more difficult to obtain the materials needed to summon it).

In hardmode, travelling to the Underground Hallow and collecting as many Crystal Shards as possible is an extremely efficient way to acquire large amounts of money, as one full stack (consisting of 250 shards) sells for 40 gold coins (16 silver coins per shard), plus it doesn't require any specific early investment or preparation (other than reaching hardmode itself). Crystals are abundant, easy to locate and mine, and respawn. Killing enemies that appear as you run around the Underground Hallow will let you collect even more coins.

A well-paying but somewhat elaborate technique is to use an obsidian generator, Bucket, several Obsidian Skin Potions, and a Hellforge: 1. Get plenty of obsidian from the generator. 2. Drink the potions and mine plenty Hellstone. 3. Combine both obsidian and Hellstone to make Hellstone Bars and sell AS IS - do not try to craft it into armor. A stack of 25 sells for about 12 gold 45 silver.

By far the easiest way is to make a chest generator by placing a chest on active stone blocks, placing at least one item in the chest and switching it to inactive. When switched to inactive it will spawn four chests. Chests can then be harvested by either hooking up a switch to the blocks and repeatedly switching the blocks from active to inactive, or if in single player mode, by attempting to place another chest on a single block next to the chest on the inactive blocks. A full stack of gold or shadow chests nets 9 gold and 90 silver. Filling your inventory takes only a few seconds and when sold yields a profit of a few platinum.

Storage
Like other items, Coins can be stored in Chests and Piggy Banks. It is recommended to always store them, as it will prevent them from being dropped upon death (Softcore mode), the penalty of which is dropping half of your carried coins (rounded up).

Inside the Inventory, Coins have their own special set of 4 slots on the right of the inventory grid.

If for some reason, you cannot store your coins, and are carrying a single high value coin (a Gold Coin early, or a Platinum Coin later), then it might be a smart move to "break" the coin: If you were to die, you would lose only half of the low currency Coins, as opposed to the entire high currency coin.

The maximum coins a player can hold with only using the coin slots is 400 platinum coins (4 slots times 100 platinum per stack). This is equivalent to four hundred million (400,000,000) copper total. If the player were to clean out all his inventory and replace them each with 100 platinum coins stacks, then the total would be 4,000 (in 40 inventory slots) plus 400 (in 4 coin slots), for a total of four thousand and four hundred platinum (4,400), which would be equivalent to four billion and four hundred million copper (4,400,000,000).

Crafting
Coins can be crafted by hand. Each coin is worth 100 of the sub denomination (Copper -> Silver -> Gold -> Platinum). A point of interest to note is that when the player picks up an item, the coins are automatically crafted if possible. This means that if the player picks up hundreds of Silver Coins, he'll end up with a few Gold Coins in his inventory, and less than 100 Silver Coins. This means that you can craft coins a lot easier (especially if you are up against a wall) by simply throwing a stack of coins out of your inventory. When you pick them up, they will be crafted for you. So if you threw 100 silver coins out of your inventory, you would pick up 1 gold instead.