ISK, or Inter-Steller Kredits are the in game currency of DUST 514 and the EVE Online universe. It is used to purchase almost any item available from the marketplace such as skill books, weapons, dropsuits and modules. ISK is used in virtually all aspects of both games flowing between the two games through contracts, trade and war. Items which cannot be bought with ISK must instead be bought with Aurum.
The primary method of aquiring ISK in DUST 514 is through combat.
Money is status, so one's worth is determined by ISK. High quality equipment can become rather expensive, and for this reason ISK is the driving force behind the progression of a mercenary. Remember though that equipment is lost upon death, so essentially each death will cost ISK if using personally funded equipment.