Not the electrical system in a small Italian car (thanks, Ian!). Broadly speaking, there are two monetary systems: commodity-based systems and fiat currency systems. The first one is composed of a commodity, credit and currency while the second is composed only of currency and credit. Historically, most currencies were pegged against a physical commodity such as gold or silver. In this type of monetary system, the government sets a value for a commodity which is exchanged for money, with the aim of tying the currency to the value of a commodity that is not easily influenced by the forces supply and demand forces – gold and silver were seen as good candidates but by no means perfect.
A fiat currency system, however, is solely based on trust and credit within an economy. In a fiat currency system, the amount of money in circulation is not constrained by the ability to exchange it for a commodity, nor is its value tied to a commodity. It rose in popularity in the 20th century and has become the dominant monetary system since the collapse of the Bretton Woods system in the early 1970s. Modern day policymakers favour fiat currency systems because they offer more flexibility over the money supply through credit creation and, in recent years, other forms of extraordinary monetary policy such as quantitative easing.