In simple words, currency trading is the act of buying and selling international currencies. Very often, banks and financial trading institutions engage in the act of currency trading. Individual investors can also engage in currency trading, attempting to benefit from variations in the exchange rate of the currencies.
The currency trading (FOREX) market is the biggest and the fastest growing market in the world economy. Its daily turnover is more than 2.5 trillion dollars, which is 100 times greater than the NASDAQ daily turnover. Markets are places to trade goods. The same goes with FOREX. The Forex goods (or merchandise) are the currencies of various countries. You buy Euro, paying with US dollars, or you sell Japanese Yens for Canadian dollars.
Word "Forex" comes from shorting up "foreign exchange" and it is used to describe currency spot market mostly but also currency futures and options market. This is the biggest and most fluent global market
Forex is over the counter market. It means that there are no agreed centres or exchanges where you need to be connected in order to trade. It is a big worldwide network, letting you trade 24 hours per day usually from Monday till Friday.