What is forex?
Forex is the market where one currency is swapped for another.
A price is a pair
EUR/USD at 1.1000 means one euro costs 1.10 US dollars. The first currency is the base, the second is the quote. If you buy EUR/USD you gain when the euro rises against the dollar, and lose when it falls.
Who trades it
Banks, companies paying for imports, funds and central banks make up most of the volume. Retail traders are a small slice and usually trade through a broker, not the market directly.
Majors, minors, exotics
Majors pair the US dollar with the euro, yen, pound, Swiss franc, Australian, Canadian or New Zealand dollar. Minors are crosses without the dollar, like EUR/GBP. Exotics pair a major with a smaller currency, like USD/ZAR. Exotics usually cost more to trade and can move sharply.
It runs around the clock
The market opens Monday morning in Sydney and closes Friday evening in New York. Activity is highest when London and New York overlap. See the session clock.