For example, you might think the Euro (EUR) is going to increase in value against the Australian dollar (AUD) so you could place a trade to buy the EUR/AUD currency pair. If the Euro rises you would make a profit; if it drops you would incur a loss. Conversely, if you thought the Euro was going to decrease in value you could place a trade that would benefit from that price movement.
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.