Just like stocks, you can trade currency based on what you think its value is (or where it's headed). But the big difference with forex is that you can trade up or down just as easily. If you think a currency will increase in value, you can buy it. If you think it will decrease, you can sell it. With a market this large, finding a buyer when you're selling and a seller when you're buying is much easier than in other markets. Maybe you hear on the news that China is devaluing its currency to draw more foreign business into its country. If you think that trend will continue, you could make a forex trade by selling the Chinese currency against another currency, say, the US dollar. The more the Chinese currency devalues against the US dollar, the higher your profits. If the Chinese currency increases in value while you have your sell position open, then your losses increase and you want to get out of the trade.
The carry trade opportunity was also seen in USD/JPY in 2005. Between January and December of that year, the currency rallied from 102 to a high of 121.40 before ending at 117.80. This is equal to an appreciation from low to high of 19%, which was far more attractive than the 2.9% return in the S&P 500 during that same year. In addition, at the time, the interest rate spread between the U.S. dollar and the Japanese yen averaged around 3.25%. Unleveraged, this means that a trader could have earned as much as 22.25% over the course of the year. Introduce 10:1 leverage, and that could be as much as 220% gain.
All forex trades involve two currencies because you're betting on the value of a currency against another. Think of EUR/USD, the most-traded 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 a price quoted on your platform, that price is how much one euro is worth in US dollars. You always see two prices because one is the buy price and one is the sell. The difference between the two is the spread. When you click buy or sell, you are buying or selling the first currency in the pair.
To trade $100,000 of currency, with a margin of 1%, an investor will only have to deposit $1,000 into her or his margin account. The leverage provided on a trade like this is 100:1. Leverage of this size is significantly larger than the 2:1 leverage commonly provided on equities and the 15:1 leverage provided in the futures market. Although 100:1 leverage may seem extremely risky, the risk is significantly less when you consider that currency prices usually change by less than 1% during intraday trading (trading within one day). If currencies fluctuated as much as equities, brokers would not be able to provide as much leverage.