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.
Currency trading was very difficult for individual investors prior to the internet. Most currency traders were large multinational corporations, hedge funds or high-net-worth individuals because forex trading required a lot of capital. With help from the internet, a retail market aimed at individual traders has emerged, providing easy access to the foreign exchange markets, either through the banks themselves or brokers making a secondary market. Most online brokers or dealers offer very high leverage to individual traders who can control a large trade with a small account balance.
Once Nixon abolished the gold standard, the dollar's value quickly plummeted. The dollar index was established to give companies the ability to hedge this risk. Someone created the U.S. Dollar Index to give them a tradeable platform. Soon, banks, hedge funds, and some speculative traders entered the market. They were more interested in chasing profit than in hedging risks. 
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 .80, which means it now costs $0.80 to buy €1.00.
Investment management firms (who typically manage large accounts on behalf of customers such as pension funds and endowments) use the foreign exchange market to facilitate transactions in foreign securities. For example, an investment manager bearing an international equity portfolio needs to purchase and sell several pairs of foreign currencies to pay for foreign securities purchases.
Forex, or the foreign exchange market (also called FX for short) is the marketplace where currencies are traded. At its simplest, a foreign exchange transaction might be, for example, when you transfer your local currency to a new one for an upcoming holiday. Across the market as a whole, an estimated 5.3 billion USD is traded every day between governments, banks, corporations, and speculators.
Some may argue that Forex day trading is a field that should be left for the people that have great experience, and who can get fully immersed in the activity. Others say that day trading is the best way to make money in the least possible time, and therefore is the best type of trading as a result of this. Whatever the case may be, day trading is the field in which both pros and cons occur. While this type of trading is gaining popularity with each passing day, it is only effective for the people who are eager to commit a lot in order to succeed at it.
More specifically, the spot market is where currencies are bought and sold according to the current price. That price, determined by supply and demand, is a reflection of many things, including current interest rates, economic performance, sentiment towards ongoing political situations (both locally and internationally), as well as the perception of the future performance of one currency against another. When a deal is finalized, this is known as a "spot deal". It is a bilateral transaction by which one party delivers an agreed-upon currency amount to the counter party and receives a specified amount of another currency at the agreed-upon exchange rate value. After a position is closed, the settlement is in cash. Although the spot market is commonly known as one that deals with transactions in the present (rather than the future), these trades actually take two days for settlement.