Every trade in the FX market involves the simultaneous purchase of a currency and the sale of another. The currency pair represents the exchange rate of the two currencies or how much of one currency (the quote currency) is needed to buy one unit of the other (base currency). If you buy a pair, you are buying the base currency and selling the quote currency.
The most traded currency is the US dollar (USD). Regarding pairs, the most popular is EUR/USD. Its liquidity conditions warrant the lowest spread of the market, making it even more attractive for active trading. It is followed by USD/JPY, GBP/USD and AUD/USD.
Volatility is critical for trading FX, so the most volatile currency pairs, usually with higher spreads, are attractive to some investors. The most volatile pairs over the last 24 months that include major currencies are AUD/JPY, NZD/JPY, CAD/JPY, GBP/NZD, GBP/AUD and EUR/AUD. If we consider exotic currencies, USD/ZAR, USD/BRL and USD/MXN offer high volatility (in pips) but their spread is considerably higher.