The number quoted for these prices is based on the current exchange rate of the currencies in the pair, or how much of the second currency you would get in exchange for one unit of the first currency (for instance, if 1 EUR could be exchanged for 1.68 USD, the bid and ask price would be on either side of this number). Learn more about Forex quotes in this article: Understanding and Reading Forex Quotes.
The foreign exchange market works through financial institutions and operates on several levels. Behind the scenes, banks turn to a smaller number of financial firms known as "dealers", who are involved in large quantities of foreign exchange trading. Most foreign exchange dealers are banks, so this behind-the-scenes market is sometimes called the "interbank market" (although a few insurance companies and other kinds of financial firms are involved). Trades between foreign exchange dealers can be very large, involving hundreds of millions of dollars. Because of the sovereignty issue when involving two currencies, Forex has little (if any) supervisory entity regulating its actions.
^ The total sum is 200% because each currency trade always involves a currency pair; one currency is sold (e.g. US$) and another bought (€). Therefore each trade is counted twice, once under the sold currency ($) and once under the bought currency (€). The percentages above are the percent of trades involving that currency regardless of whether it is bought or sold, e.g. the U.S. Dollar is bought or sold in 88% of all trades, whereas the Euro is bought or sold 32% of the time.
With the expansion of retail brokers (which, of course, should always be regulated), the population size of intraday traders operating in a specific intraday time frame (M1-H1) determines the profitability of the trader trading this time frame. As the number of traders trading certain intraday time frames increases, conversely, the competition also increases, and the markets may become more efficient and easier to trade.
Forex is a portmanteau of foreign currency and exchange. Foreign exchange is the process of changing one currency into another currency for a variety of reasons, usually for commerce, trading, or tourism. According to a recent triennial report from the Bank for International Settlements (a global bank for national central banks), the average was more than $5.1 trillion in daily forex trading volume.