SO about the tv show LOSt?

I'm confused so I'm on season 3 I'm watching it on netflix...ialready know they all are dead.but DO they actually survive the Plane crash??? And then die??? To where they think they left the island and live "SIDEWAY world? Which is just a purtgory right..but this sideway world??? Is the flash backs I'm seeing now their sideway world?? Or they don't start showing the Sideways world till a certain season??? So everysingle person on the island that they ever showed is dead?? Or are they alive on the island???and what the heck is with all this dharma thing??? And the other ughh..very confusing ..can anyone fill me in..I allready know the end I just want to understand..and is this WHOLE show bout Jack find redemtion??