There are two types of currency
Energon - Dropped by defeated enemies, mined with an Energon Probe(1) or earned by completing missions. You must deposit collected Energon into Energon Deposit sites and interacting with the default key F and holding until your blue Energon meter is delpeted. This is important because if you die you will lose all Energon not in your main reserve
Relics - Bought with real money through the store ( credit card, Paypal, Jagex Game Studios Pre-Paid Game Cards, ect.), or earned slowly defeating enemies and completing challenges and missions.