JEE is the entrance examination for admission to Engineering colleges across India and JEE Main score is the gate way for admission to IITs, NITs and GFTIs across India. So if you want to get into any Engineering college, you need to appear for JEE Mains. But if you want to get admission to any of the IITs, NITs and GFTIs, you need to appear for JEE Advanced. And for appearing for JEE Advanced, you need to qualify in JEE Mains.