Historically, different international monetary systems have emphasized different policy mixes. For instance, the Bretton Woods system emphasized the first two at the expense of free capital movement. The collapse of the system destroyed the stability and predictability of the currency markets. The resultant large fluctuations meant a rise in exchange rate risk (as well as in profit opportunities). Governments now face numerous challenges that are often captured under the term globalization or capital mobility: the move to floating exchange rates, the political liberalization of capital controls, and technological and financial innovation.
Either way you don’t have to provide the full currency value to open your position. Instead you put down a margin deposit, which is a fraction of the full value. And you don’t actually buy or sell any currency: you are opening a speculative position on the change in value of the forex pair. Your profit or loss is realised when you close your position by selling or buying.
When you trade forex, you're effectively borrowing the first currency in the pair to buy or sell the second currency. With a US$5-trillion-a-day market, the liquidity is so deep that liquidity providers—the big banks, basically—allow you to trade with leverage. To trade with leverage, you simply set aside the required margin for your trade size. If you're trading 200:1 leverage, for example, you can trade £2,000 in the market while only setting aside £10 in margin in your trading account. For 50:1 leverage, the same trade size would still only require about £40 in margin. This gives you much more exposure, while keeping your capital investment down.
There are actually three ways that institutions, corporations and individuals trade forex: the spot market, the forwards market and the futures market. The forex trading in the spot market always has been the largest market because it is the "underlying" real asset that the forwards and futures markets are based on. In the past, the futures market was the most popular venue for traders because it was available to individual investors for a longer period of time. However, with the advent of electronic trading and numerous forex brokers, the spot market has witnessed a huge surge in activity and now surpasses the futures market as the preferred trading market for individual investors and speculators. When people refer to the forex market, they usually are referring to the spot market. The forwards and futures markets tend to be more popular with companies that need to hedge their foreign exchange risks out to a specific date in the future.