Currency is an item used to purchase items. Aside from money, there are several types of currencies, player can gain them by participating on events (like favor from PvP events or planar currencies from rifts and invasions), by collecting artifacts or by crafting items.
The most common currency is Platinum, Gold and Silver. Money conversion:
Artisan Mark is a reward for completing workorders - daily quests to craft certain amount of items. The amount and goal of workorders depends on player's crafting skill level. Players can buy additional recipes for artisan marks at recipe vendors in capital cities.
These currencies can be gained by defeating invasions, closing rifts or destroying footholds. The most common currency is planarite. Other currencies are zone based and can be of epic or rare quality. Planar goods vendors are in each zone, they sell equipment, planar focuses and essences. It is a good practice to spend zone based planar currency here as soon as possible.
Lucky Coins can be gained by turning collections at artifact collectors in capital cities, and players can use them to buy vanity pets or items.
Otherworldly Sourcestones could be gained by closing Death Rifts or doing Daily Quest during the Grim Harvest of the 1.1 Update. They could be exchanged for select rewards at the Event seller.