The Forex market (foreign exchange market) is useful because it helps enable trade and transactions between countries, and it also allows an investment opportunity for risk seeking investors who don't mind engaging in speculation.
Regarding the specifics of buying and selling on forex, it is important to note that currencies are always priced in pairs. All trades result in the simultaneous purchase of one currency and the sale of another.
This necessitates a slightly different mode of thinking than what you might be used to. While trading on the forex, you would execute a trade only at a time when you expect the currency you are buying to increase in value relative to the one you are selling.
If the currency you are buying does increase in value, you must sell the other currency back in order to lock in a profit. An open trade (or open position), therefore, is a trade in which a trader has bought or sold a particular currency pair and has not yet sold or bought back the equivalent amount to close the position.
Selling and Buying Currencies on the Forex / foreign exchange