What is the difference between oxycontin and oxycodone?

My mom is going thru cancer and has been on percocet (oxycodone + acetaminophen) for a year and recently her oncologist put her on oxycontin every 12 hours and percocet for breakthrough pain as needed. Is oxycontin just a long term or delayed release form of oxycodone or something different entirely? We just want to understand what she's on.