There is no integrated market for the bulk of forex trading, and regulations about it are little. There are several unified marketplaces globally because of the over-the-counter feature of the currency markets. This means that various rates or prices exist instead of a single one, and it depends on the location and who are the trading parties, i.e. bank or maker of market. In practice, expect that the rates are very close, to avoid the abuse by arbitrageurs immediately. A specific quoted price currency is typically the London market price because the London market dominates it.
London, Hong Kong, New York, Tokyo, and Singapore are all significant forex trading centers. Banks all over the world partake in the trading that happens constantly 24 hours a day, except weekends.
There is always fluctuations in forex, and the typical cause are the flow of actual money, the expected changes in the flow of money caused by alterations in the growth of gross domestic product (GDP) growth, inflation rates, interest rates, deficits and surpluses of budget and trade, big cross-border deals in M&A and other macro-economic circumstances.
The trading of currencies is against to another currency. Every pair of currency makes up an entity of trading product and conventionally distinguished by YYYZZZ or YYY/ZZZ, where the YYY and ZZZ naming comes from the standardized code set by the ISO 4217. The YYY is the first currency and acts as the base currency quoted relative to the ZZZ, which is the second currency and termed as the counter or quoted currency. Any factors that affect the base currency will also affect the counter currency.
Some trivia about forex trading: The most traded products in the foreign exchange market are EURUSD (accounts for 27%), USDJPY (13%), and GBPUSD (12%). The currency of US account for 86.3% of transactions involved next is the euro (37.0%), followed by the yen (17.0%), and then the sterling (15.0%). The percentages of volume for all currencies should be the sum of 200% because two currencies transacts.