Foreign exchange market

The foreign exchange (currency or forex or FX) market exists wherever one currency is traded for another. It is by far the largest financial market in the world, and includes trading between large banks, central banks, currency speculators, multinational corporations, governments, and other financial markets and institutions. The average daily trade in the global forex markets currently around US$ 1.9 trillion.^[1] Retail traders (individuals) are a small fraction of this market and may only participate indirectly through brokers or banks.

source:- WIKI

Retail forex brokers

Retail forex brokers or market makers handle a minute fraction of the total volume of the foreign exchange market. According to CNN, one retail broker estimates retail volume at $25–50 billion daily, which is about 2% of the whole market and it has been reported by the CFTC website that unexperienced investors may become targets of forex scams.

source :- wiki

Banks

The interbank market caters for both the majority of commercial turnover and large amounts of speculative trading every day. A large bank may trade billions of dollars daily. Some of this trading is undertaken on behalf of customers, but much is conducted by proprietary desks, trading for the bank's own account.

Until recently, foreign exchange brokers did large amounts of business, facilitating interbank trading and matching anonymous counterparts for small fees. Today, however, much of this business has moved on to more efficient electronic systems, such as EBS (now owned by ICAP), Reuters Dealing 3000 Matching (D2), the Chicago Mercantile Exchange, Bloomberg, and TradeBook(R). The broker squawk box lets traders listen in on ongoing interbank trading and is heard in most trading rooms, but turnover is noticeably smaller than just a few years ago.

source :- wiki