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.
The main trading centers are London and New York City, though Tokyo, Hong Kong, and Singapore are all important centers as well. Banks throughout the world participate. Currency trading happens continuously throughout the day; as the Asian trading session ends, the European session begins, followed by the North American session and then back to the Asian session.
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.
Unlike stock markets, which can trace their roots back centuries, the forex market as we understand it today is a truly new market. Of course, in its most basic sense - that of people converting one currency to another for financial advantage - forex has been around since nations began minting currencies. But the modern forex markets are a modern invention. After the accord at Bretton Woods in 1971, more major currencies were allowed to float freely against one another. The values of individual currencies vary, which has given rise to the need for foreign exchange services and trading.