A long position is one in which you buy a currency at one price, with the expectation of selling it later on at a higher price. Obviously, you anticipate that the market will rise. A short position is one in which you sell a currency with the expectation of buying it back at a lower price. Here, you expect the market to fall. Every FX position you take automatically entails going long in one currency, and short the other. If you buy one, by default you are shorting the other.