A "currency pair" is a set of two or more currencies. It's written as "US dollar / yen" for the US dollar and the yen, "euro / yen" for the euro and the yen, and "euro / US dollar" for the euro and the US dollar.
There are various currency pairs in the forex market because many countries and areas throughout the world have their own currencies.
The "exchange rate" is the ratio (price) utilized when converting one currency for another.
If the currency is the US dollar / yen, the exchange rate is "1 US dollar = 100 JPY."
In this situation, one US dollar can be traded for 100 yen, and one US dollar for 100 yen.
As the value of each other's currencies grows and decreases, the "exchange rate" varies from moment to moment. The "exchange rate" for US dollars / yen may be "1 US dollar = 100 yen," "1 US dollar = 110 yen," or "1 US dollar = 90 yen."
