An investor can profit from the difference between two interest rates in two different economies by buying the currency with the higher interest rate and shorting the currency with the lower interest rate. Prior to the 2008 financial crisis, it was very common to short the Japanese yen (JPY) and buy British pounds (GBP) because the interest rate differential was very large. This strategy is sometimes referred to as a "carry trade."
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.
All forex transactions include two currencies, because you are betting on the value of one currency against another. Think of EUR / USD, the best-selling currency pair in the world. EUR, the first currency in the pair, is the base, and USD, the second, is the counter. When you see the price indicated on your platform, this price is equal to the value of one euro in US dollars. You always see two prices, because one is the purchase price and the other is the sale. The difference between the two is in distribution. When you click buy or sell, you buy or sell the first currency in a pair.
The blender costs $100 to manufacture, and the U.S. firm plans to sell it for €150—which is competitive with other blenders that were made in Europe. If this plan is successful, the company will make $50 in profit because the EUR/USD exchange rate is even. Unfortunately, the USD begins to rise in value versus the euro until the EUR/USD exchange rate is 0.80, which means it now costs $0.80 to buy €1.00.
Since the market is made by each of the participating banks providing offers and bids for a particular currency, the market pricing mechanism is based on supply and demand. Because there are such large trade flows within the system, it is difficult for rogue traders to influence the price of a currency. This system helps create transparency in the market for investors with access to interbank dealing.