Okay the version i heard was the X-men were in space and they had a proublem and someone had to take the wheel and Jean was the only one that could do it and the radiation caused Jean to become Pheonix or somthing like that then later after Pheonix saved the world by taking the gem she the Pheonix was guardian of somthing happened and she became the dark pheonix and i'm not sure about what happened after that my memory is a little fuzzy but if your asking about the movie version then thats a totally different story in the movie she saved the x-mens lives by blocking the water from the burst bridge and made the xjet take off without her but not to the xmens knolege she didn't die and in the last stand they find her and according to the professor her powers put her into a knetic chrisalis and later we find out that the dorment side of jeans mind is the pheonix and the pheonix becomes evil and joins magneto and in the end wolverine had to kill her to save everyone

This video explains it well. :)