Foreign exchange markets play an important role in the global economy. This important role in the economy becomes more clear when take a loop at transaction volumes of securities. In contrast to what many people think and to what is mostly covered in the news, currencies (not shares) are mostly traded in the world. In what follows we explain some of important concepts related to foreign exchange markets which should be clearly understood in order to invest in securities denominated in foreign currencies or enacting forex trading strategies. We begin by explaining one of the most important and simple features called triangular arbitrage.