In terms of volume of trading, it is by far the largest market in the world.
The foreign exchange market determines the relative values of different currencies. The foreign exchange market works through financial institutions, and it operates on several levels. Behind the scenes banks turn to a smaller number of financial firms known as “dealers,” who are actively 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. Because of the sovereignty issue when involving two currencies, Forex has little (if any) supervisory entity regulating its actions.
But because there is little or no oversight by US regulators doesn't mean it's illegal or a scam. It does mean you have to do a little more research for a good forex broker; scams are nothing new in the finance industry.
You should get brief knowledge about Forex Brokers that accept US clients