Definition of different economy used on the server.
There are multiple economies on the server, the main one used outside of specialised systems is just called your wallet, balance or coins.
Wallet / Coins
Wallet / Coins usually identified by a gold coin, this is the default currency for everything. Check out Auction House.
Foreign Gems are identifiable by a blue gem stone icon and are exclusively used for the Elite system, This is the currency that drops from Elite mobs, Elite events, Elite quests and in dungeons. You can purchase Elite gear from special venders and use them to level up your guild rank.