Coins are a form of universal currency used in the IndustrialCraft mod, intended for use with the Trade-O-Mat vending machines. Like the Trade-o-Mats themselves, Coins are only sensible to use on a Multiplayer server. Four units of Refined Iron can be crafted into sixteen IC Coins, which can then be put in a Trade-o-Mat programmed to receive them as payment. The value of each coin is determined by the 'prices' set on the Trade-o-Mat by the player that built it.
Additionally, a player can establish a Trade-o-Mat 'banking system'; a series of Trade-o-Mat machines which only dispense IC Coins in exchange for certain resources. This would, in essence, set the basis of a Minecraft economy using an 'Iron Standard', with each coin being worth one-quarter of a Refined Iron Ingot. For this reason, the coins are often colloquially referred to as 'Quarters', due to their value, and their general appearance.
The value of other materials can be set according to how much easier/harder they are to find or craft, in comparison to Refined Iron.
Iron to Coins
Coins to Iron