In every foreign exchange transaction, you are simultaneously buying one currency and selling another. In effect, you are using the proceeds from the currency you sold to purchase the currency you are buying. Furthermore, every currency in the world comes attached with an interest rate set by the central bank of that currency's country. You are obligated to pay the interest on the currency that you have sold, but you also have the privilege of earning interest on the currency that you have bought. For example, let’s look at the New Zealand dollar/Japanese yen pair (NZD/JPY). Let’s assume that New Zealand has an interest rate of 8% and that Japan has an interest rate of 0.5% In the currency market, interest rates are calculated in basis points. A basis point is simply 1/100th of 1%. So, New Zealand rates are 800 basis points and Japanese rates are 50 basis points. If you decide to go long NZD/JPY you will earn 8% in annualized interest, but have to pay 0.5% for a net return of 7.5%, or 750 basis points.
A single pound on Monday could get you 1.19 euros. On Tuesday, 1.20 euros. This tiny change may not seem like a big deal. But think of it on a bigger scale. A large international company may need to pay overseas employees. Imagine what that could do to the bottom line if, like in the example above, simply exchanging one currency for another costs you more depending on when you do it? These few pennies add up quickly. In both cases, you—as a traveler or a business owner—may want to hold your money until the forex exchange rate is more favorable.
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.

It is hard to determine the best level one should use, as it mainly depends on the trader's strategy and the actual vision of upcoming market moves. As a rule of thumb, the longer you expect to keep your position open, the smaller the leverage should be. This would be logical, as long positions are usually opened when large market moves are expected. However, when you are looking for a long lasting position, you will want to avoid being 'Stopped Out' due to market fluctuations.
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.