Basic currency in game, you will buy health potions, injury kits and many other important items required for exploration.
Gold, g, 1g = 100s
Silver, s, 1s = 100c
Copper, c, 10000c = 1g
Gold will drop from most enemies you will kill, there is no direct way to exchange gold for other currencies, but you can for example invest in some items and sell them in Auction House for Diamonds.
Examples of where the you will pay with gold for various items:
Vendors accept gold as a currency, you can buy here health potions, skill kits, basic equipment, etc.
Companions shop, among many other currencies this vendor will accept gold.
You will pay with gold for profession resources..
Basic mounts are bought for gold.