Everyday, currencies are traded in an international foreign exchange market, otherwise known as the forex market, with the main marketplaces (otherwise known as bourses) existing in the world's financial centes New York, London, Tokyo, Frankfurt and Zurich. Historically, the only way to participate was from the trading floor of one of these bourses, but today, people can trade forex from anywhere through a secure internet connection and a PC.
Today's traders operate in a global network, taking positions in the market and making investment decisions based on either relative value between two currencies, or a particular currency's actual price. Currency value fluctuations are constantly renegotiated through trading activity, and this activity, and the corresponding currency values are also indicators of the levels of currency supply.
An example of market behaviour greater demand for the Euro might indicate a weakening supply. Low supply and increased demand will drive the price of the Euro up against other currencies like the dollar, until the price better reflects what traders are prepared to pay when short supply exists. Another way to look at this situation is this higher demand means it will cost more dollars to buy the Euro, which equates to a weakening of the dollar in comparison. Analysis of situations such as in this example forms the basis for a trader's investment decisions, and they will purchase or sell currency accordingly.
This should be remembered, as while many see the foreign exchange market as the vehicle for converting their home currency while travelling abroad, many others choose to use the market to advance their financial position and secure their future.