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. Ale is even more profitable, as it turns 2 sand into 20 copper for an exchange of 10 copper per unit of sand.

An even better way, however needing a few items such as a Bucket and a Nightmare Pickaxe, is to use an Obsidian Generator to mass-produce Obsidian Skulls. These sell for 54 silver each, so selling two of them already gives you over 1 Gold Coin. With this technique, it is possible to acquire the Minishark in half an hour.

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

Repeatedly killing the Eye of Cthulhu will yield an average of about 50 Demonite Ore each time, which sells for 8 silver each, which adds up to 4 gold a run.

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.

Another one requires an obsidian generator, a Bucket, as well as several Obsidian Skin Potions.

Step 1: Get loads of obsidian from your generator.

Step 2: Drink an Obsidian Skin Potion and mine loads of Hellstone.

Step 3: Make loads of Hellstone Bars and sell as is. A 25 stack sells for about 12 gold 45 silver whereas a molten helmet that requires 25 bars sells for only 90 silver.

Another technique is to fight the eater of worlds. By destroying 2 orbs and wait for the 3rd one. When you have found the the 3rd orb, make worm food, that allowes you to fight him again after smashing the 3rd orb and kill the eater of worlds, this method will make about 20 gold coins if everything is collected.

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 have with only using the coin slots is ten, (1x100x100x100) to make the platinum coin and then times that by another 100 to make 10,000,000 (Ten million). Now multiply that by 4 to get your maximum coinslots. This makes forty million coins total. If the player were to clean out all his inventory and replace them each by 100 platinum coins then the total would be 1x100x100x100x100x40 = Fourhundred million, or 400,000,000. Add these together and the total would be 440,000,000, or 4.4*10^8.

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.