Retail forex brokers

There are two types of retail brokers offering the opportunity for speculative trading. Retail forex brokers or Market makers.Retail traders (individuals) are a small fraction of this market and may only participate indirectly through brokers or banks.
Retail forex brokers, while largely controlled and regulated by the CFTC and NFA might be subject to forex scams. At present, the NFA and CFTC are imposing stricter requirements, particularly in relation to the amount of Net Capitalization required of its members. As a result many of the smaller, and perhaps questionable brokers are now gone. It is not widely understood that retail brokers and market makers typically trade against their clients and frequently take the other side of their trades. This can often create a potential conflict of interest and give rise to some of the unpleasant experiences some traders have had. A move toward NDD(No Dealing Desk), And STP(Straight Through Processing) has helped to resolve some of these concerns and restore trader confidence, but caution is still advised in ensuring that all is as it is presented.

Source: http://en.wikipedia.org/wiki/Foreign_exchange_market

Central banks

National central banks play an important role in the foreign exchange markets. They try to control the money supply, inflation, and/or interest rates and often have official or unofficial target rates for their currencies. They can use their often substantial foreign exchange reserves to stabilize the market. Milton Friedman argued that the best stabilization strategy would be for central banks to buy when the exchange rate is too low, and to sell when the rate is too high — that is, to trade for a profit based on their more precise information. Nevertheless, the effectiveness of central bank "stabilizing speculation" is doubtful because central banks do not go bankrupt if they make large losses, like other traders would, and there is no convincing evidence that they do make a profit trading.
The mere expectation or rumor of central bank intervention might be enough to stabilize a currency, but aggressive intervention might be used several times each year in countries with a dirty float currency regime. Central banks do not always achieve their objectives. The combined resources of the market can easily overwhelm any central bank. Several scenarios of this nature were seen in the 1992–93 ERM collapse, and in more recent times in Southeast Asia.