Money serves three main uses: as a medium of exchange to determine value during transactions, as a unit of account to compare the values of goods and services, and as a store of value that generally maintains its purchasing power even if saved over time. For something to function as money, it must meet six key characteristics: durability, portability, divisibility, uniformity, limited supply, and general acceptability. The value of money comes from it representing value in itself (commodity money), being exchangeable for something else of value (representative money), or having value by government decree (fiat money).