ISO (International Organization for Standardization)
Established in 1947, ISO established a standard for country and currency pair abbreviations. Since foreign currencies are quoted in terms of value of one currency against another, a currency pair consists of an acronym for both currencies, separated by a slash "/".
Market increments are measured in PIPs, or Percentage in Point. A pip is the last digit in the value of a currency pair; 1.3294, 115.13 etc. All currency pairs, except for the Japanese Yen, have 4 decimal places. The Yen crosses only have 2 decimal places.
For example, let's assume a trader buys 1 standard lot of GBP/USD. The current market rate is 1.9615. Essentially this trader is buying EU 100,000 in exchange for $196,150. Again, for examples sake, assume the market rate rose 15 PIPs to 1.9630 and the trader liquidates the position. The same Bu 100,000 is now worth $196,300, the trader realizing a $150 profit.
Some currency pairs are traded more heavily than others. The currency pairs that have the most volume consist of the "majors". While it can be debated which currency pairs are considered a major, it can be widely agreed upon that the following 6 pairs are: