In 1876, something called the gold exchange standard was implemented. Basically it said that all paper currency had to be backed by solid gold; the idea here was to stabilize world currencies by pegging them to the price of gold. It was a good idea in theory, but in reality it created boom-bust patterns which ultimately led to the demise of the gold standard.
For trading purposes, the first currency listed in the pair is always the directional currency on a forex price chart. If you pull up a chart of the EUR/USD, and the price is moving higher, it means the EUR is moving higher relative to the USD. If the price on the chart is falling, then the EUR is declining in value relative to the USD. The attached chart shows this.