In return for executing your buy and sell orders, forex brokers will either take a commission per trade or a spread. A spread is the difference between the bid price and ask price for the trade. The asking price is the lowest price that a currency pair will be offered for sale and the bidding price is always lower. When forex brokers successfully execute a buy with the lower bid, they will take that difference — the spread — as payment.
A swap trade involves both. Dealers buy a currency at today's price on the spot market and sell the same amount in the forward market. This way, they have just limited their risk in the future. No matter how much the currency falls, they will not lose more than the forward price. Meanwhile, they can invest the currency they bought on the spot market.
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.