Gold is a secondary currency used to purchase premium items. Credits, also known as silver, are earned in battle or purchased with gold. Credits are the primary in-game currency in our games and cannot be converted to gold. However the opposite is true, you can convert gold into credits.
Premium options bought with gold will neither change the gameplay significantly, nor give players a decisive advantage in battle. Consumables and premium ammunition can be purchased with credits.
You can find more information about this topic on the Wiki.