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, finding in chests or trading with NPCs (or other players). Even though the Coins are "Copper", "Silver", "Gold" or "Platinum", 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 99 silver and 90 copper per full stack).

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).

Another quite easy way is to make another world, go to the Corruption, locate a Shadow Orb and then use a hammer to break the Shadow Orbs, selling the items you get from them (which is optional). Once you have broken 3 Shadow Orbs the Eater Of Worlds will spawn. Kill him and collect everything. Turn the Demonite Ore into bars, then sell them. One stack of 99 Demonite Bars can sell for 31 Gold Coins. Each world spawns with about 6-10 Shadow Orbs, therefore spawning about 3 Eater Of Worlds, which, in total, would drop about one or two Platinum Coins and a few Gold Coins.

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.

In hardmode, traveling 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.

If you have already progressed far into Hardmode, farming The Destroyer for his Hallowed Bars (as of 1.2) can make a really high profit. Just craft the Hallowed Bars into a Gungnir over and over again and sell those. A Gungnir without any prefixes sells for 30 Gold, and considering the ease with which one can beat The Destroyer, it's only a matter of acquiring enough Mechanical Worms.

If you have a fish statue and some wire, hook up the statue and grind the goldfish. You can get gold fish, Jellyfish necklaces, and rarely, fish banners. Selling the goldfish at 1 silver per fish is quite good and a jellyfish necklace sells for 1 gold coin.

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.

Coins stored in the trash slot will not be dropped upon death. This is a good way to store your highest value coins if you feel you are about to die. You can then take it out of the trash slot when you respawn. If you accidentally place something else in the trash, though, the coins will be lost.

You can also get Gold Coins from lost loot in caves. They look like bags with coins flowing out of them.

Crafting
On the PC version, 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.