There are 4 types of currency:
Credits: This is in-game earned currency. It is used to repair vehicles, purchase standard vehicles, purchase ammo and module, and purchase and mount equipment.
Gold: This currency can only be earned in special missions, events, or purchased via premium shop. This is used to purchase premium vehicles, convert into other currencies, retrain crews (to 100% or change roles), and demount complicated equipment
Free experience: This is earned from battle rewards and from missions. It can be used to speed up training of perks and research any modual on tanks.
Bonds: Earned by playing ranked battles, or earning certain medals. These are used to purchase improved equipment, demount improved equipment, and buy directives.
Credits are the most important. Premium vehicles can generate credits in vast amounts (the higher the tier, the greater). Low tier vehicles (Specifically tiers IV or lower) can generate credits no matter what the battle turns out. Keep in mind that some vehicles are hard to maintain (like T32). To maximize battle income, maximize direct and spot damage and use cheap ammo/consumables.