Currencies are traded on the Foreign Exchange market, also known as Forex. This is a decentralized market that spans the globe and is considered the largest by trading volume and the most liquid worldwide. Exchange rates fluctuate continuously due to the ever changing market forces of supply and demand. Forex traders buy a currency pair if they think the exchange rate will rise and sell it if they think the opposite will happen. The Forex market remains open around the world for 24 hours a day with the exception of weekends.
Despite being able to trade 24 hours a day, 5 days a week, you shouldn’t (Forex trading is not quite 24.7). You should only trade a forex pair when it’s active, and when you’ve got enough volume. Trading forex at weekends will see small volume. Take GBP/USD for example, there are specific hours where you have enough volatility to create profits that are likely to negate the bid price spread and commission costs.
Many currency pairs will move about 50 to 100 pips per day(sometimes more or less depending on overall market conditions). A pip (an acronym for Point in Percentage) is the name used to indicate the fourth decimal place in a currency pair, or the second decimal place when JPY is in the pair. When the price of the EUR/USD moves from 1.3600 to 1.3650, that's a 50 pip move; if you bought the pair at 1.3600 and sold it at 1.3650, you'd make a 50-pip profit.

Controversy about currency speculators and their effect on currency devaluations and national economies recurs regularly. Economists, such as Milton Friedman, have argued that speculators ultimately are a stabilizing influence on the market, and that stabilizing speculation performs the important function of providing a market for hedgers and transferring risk from those people who don't wish to bear it, to those who do.[79] Other economists, such as Joseph Stiglitz, consider this argument to be based more on politics and a free market philosophy than on economics.[80]
How much each pip is worth is called the "pip value." For any pair where the USD is listed second, the above-mentioned pip values apply. If the USD is listed first, the pip value may be different. To find the pip value of the USD/CHF, for example, divide the normal pip value (mentioned above) by the current USD/CHF exchange rate. A micro lot is worth $0.10/0.9435 = $0.1060, where 0.9435 is the current price of the pair. For JPY pairs (USD/JPY), go through this same process, but then multiply by 100. For a more detailed explanation, see Calculating Pip Value in Different Forex Pairs.
Unlike stock markets, which can trace their roots back centuries, the forex market as we understand it today is a truly new market. Of course, in its most basic sense—that of people converting one currency to another for financial advantage—forex has been around since nations began minting currencies. But the modern forex markets are a modern invention. After the accord at Bretton Woods in 1971, more major currencies were allowed to float freely against one another. The values of individual currencies vary, which has given rise to the need for foreign exchange services and trading.