It may have intrinsic value (commodity money), be legally exchangeable for something with intrinsic value (representative money), or only have nominal value (fiat money).
In his book Debt: The First 5,000 Years, anthropologist David Graeber argues against the suggestion that money was invented to replace barter.
The psychological elements required for a barter system to exist and function are not only far simpler than those required for a simple monetary system, but every historical example of monetary systems requires and incorporates the basic psychological elements that make barter possible.
It is very simple to trade a piece of meat for a piece of fruit (even a caveman can do it), but to create and use tools to make hash marks on a stick or piece of leather to represent that someone gave you a piece of meat and now you owe them a piece of meat is a far more complex set of thought processes requiring a knowledge of making tools, knowledge of crude numerical systems, etc..
For example, a farmer has to find someone who not only wants the grain he produced but who could also offer something in return that the farmer wants.
Finding people to barter with is a time-consuming process and this factor is most likely the main driving force in the creation of monetary systems – people seeking a way to stop wasting their time looking for someone to barter with.
The Commodity theory is more widely held and much of this article is written from that point of view.) the Greek philosopher Aristotle contemplated the nature of money.