When trading Forex, you'll see that both 'Bid' and 'Ask' prices are quoted. The bid price is the price at which you can buy the currency, while the ask price is the price at which you can sell it. If you are purchasing a currency in a trade, this is known as a long trade, and the hope is that the currency pair will increase in value, so that you can sell it at a higher price and make a profit on the difference.
The foreign exchange market is where currencies are traded. Currencies are important to most people around the world, whether they realize it or not, because currencies need to be exchanged in order to conduct foreign trade and business. If you are living in the U.S. and want to buy cheese from France, either you or the company that you buy the cheese from has to pay the French for the cheese in euros (EUR). This means that the U.S. importer would have to exchange the equivalent value of U.S. dollars (USD) into euros. The same goes for traveling. A French tourist in Egypt can't pay in euros to see the pyramids because it's not the locally accepted currency. As such, the tourist has to exchange the euros for the local currency, in this case the Egyptian pound, at the current exchange rate.