Trade involves the transfer of goods or services from one person or entity to another, often in exchange for money. A network that allows trade is called a market.
The original form of trade, barter, saw the direct exchange of goods and services for other goods and services. Barter is trading things without the use of money. Later one side of the barter started to involve precious metals, which gained symbolic as well as practical importance. Modern traders generally negotiate through a medium of exchange, such as money. As a result, buying can be separated from selling, or earning. The invention of money (and later credit, paper money and non-physical money) greatly simplified and promoted trade. Trade between two traders is called bilateral trade, while trade between more than two traders is called multilateral trade.