Money by Wikipedia

Money is anything that is generally accepted as payment for goods and services and repayment of debts. The main functions of money are as a medium of exchange, a unit of account, and a store of value. In the past, money was almost always commodity money, money with intrinsic value from the commodity of which it is made (eg. gold and silver coins, shells, tobacco leaves, grain). However, modern monetary systems are based on fiat money – money without intrinsic value, but declared by a government to be legal tender, that is, it must be accepted as a form of payment for 'all debts, public and private'. The term "price system" is sometimes used to refer to methods using commodity valuation or money accounting systems.
The money supply of a country is usually held to consist of currency (banknotes and coins) and 'deposit money' (the balance held in checking accounts and savings accounts). These demand deposits usually account for a much larger part of the money supply than currency. Deposit money is intangible and exists only in the form of various bank records. Despite being intangible, deposit money still performs the basic functions of money, as checks are generally accepted as a form of payment and as a means of transferring ownership of deposit money.
Money is generally considered to have the following functions, which are summed up in a rhyme found in older economics textbooks: "Money is a matter of functions four, a medium, a measure, a standard, a store." That is, money functions as a medium of exchange, a unit of account, a Standard of deferred payment, and a store of value. However, most modern textbooks now list only three functions, that of medium of exchange, unit of account, and store of value.
There have been many historical disputes regarding the combination of money's functions, some arguing that they need more separation and that a single unit is insufficient to deal with them all. One of these arguments is that the role of money as a medium of exchange is in conflict with its role as a store of value: its role as a store of value requires holding it without spending, whereas its role as a medium of exchange requires it to circulate. Others argue that storing of value is just deferral of the exchange, but does not diminish the fact that money is a medium of exchange that can be transported both across space and time. The term 'financial capital' is a more general and inclusive term for all liquid instruments, whether or not they are a uniformly recognized tender.

Medium of exchange
Main article: Medium of exchange
When money is used to intermediate the exchange of goods and services, it is performing a function as a medium of exchange. It thereby avoids the inefficiencies of a barter system, such as the 'double coincidence of wants' problem.

Unit of account
Main article: Unit of account
A unit of account is a standard numerical unit of measurement of the market value of goods, services, and other transactions. Also known as a "measure" or "standard" of relative worth and deferred payment, a unit of account is a necessary prerequisite for the formulation of commercial agreements that involve debt. To function as a 'unit of account', whatever is being used as money must be:
Divisible into smaller units without loss of value; precious metals can be coined from bars, or melted down into bars again.
Fungible: that is, one unit or piece must be perceived as equivalent to any other, which is why diamonds, works of art or real estate are not suitable as money.
A specific weight, or measure, or size to be verifiably countable. For instance, coins are often made with ridges around the edges, so that any removal of material from the coin (lowering its commodity value) will be easy to detect.

Store of value
Main article: Store of value
To act as a store of value, a commodity, a form of money, or financial capital must be able to be reliably saved, stored, and retrieved — and be predictably useful when it is so retrieved. Fiat currency like paper or electronic money no longer backed by gold in most countries is not considered by some economists to be a store of value.