To keep it short,Jean Grey was exposed to solar radiation whilst piloting a damaged space shuttle back to Earth after an X-men mission. She was burnt up by this radiation and she "died". This radiation however at the same time unleashed her full genetic potential and she was reborn as Phoenix. A vastly powerful cosmic being and a name symbolic of her death and rebirth in and from fire. Jean used this power to save the multiverse from the power of the M'Kraan crystal. In the Dark Phoenix Saga Jeans vast power later corrupted her and she killed billions and went on to commit suicide as a way of putting a stop to any future damage she could potentially do if she lost control again. Years later writers put together a team called X-Factor which they wanted to feature the founding members of the X-men, Cyclops, Iceman, Beast, Angel and Jean. The problem was that Jean was dead, plus even if she was brought back to life she was a mass murderer and therefore not exactly hero material anymore. The writers got around this by introducing a retcon to the Phoenix. Instead of Phoenix being Jean who had reached her ultimate potential and had become one with the power of creation, they introduced something called the Phoenix Force, which they said had seen Jeans plight on the space shuttle and offered to save her life and her friends if Jean would allow it to take her place and experience human life. This retcon meant that it was this new Phoenix Force that had paraded around as Phoenix and it was this Phoenix Force that was now accountable for the crimes of Dark Phoenix. Jean Grey was then brought back into the fold free from taint. This retcon happened in 1986 and so has been in place for most of the time that the Phoenix character has existed in comics. (Phoenix was introduced in 1980). Therefore its the interpretation most widely known by readers. However in 2003's New X-men (version 1) Grant Morrisson paved the way for the scrapping of this retcon by again making Jean and Phoenix one. Phoenix was again Jean realising her full genetic potential and the crimes of Dark Phoenix in the past were once again on her shoulders as per comic continuity. Greg Pak supported Morrisson in establishing this original interpretation as continuity again with the 2005 released Phoenix Endsong where it was reiterated that Jean is the Phoenix Force made flesh, she is not a separate entity but a part of the Force. The recently started mini Phoenix Warsong continues the Phoenix story.

From what I got of it, the Pheonix is known as the 'Devourer of Worlds', engulfing and destroying planets one by one. However, at some point for some reason or other it needed a human host. Jean Gray, in the process of saveing Earth, ended up dieing. The Pheonix discovered Jean Gray and how strong she was. Being strong enough to not be destroyed by the Pheonix entity, it aproached her and offered itself to her. After taking it within her, she became possessed, though able to control it in the beginning. After a while, it took over. That's really it unless you want to go on into the whole clones and repossession and ressurection. And all of that depends on which time line you're following after Marvel got greedy and began to split the series willy-nilly.

I have no idea but I'm sure there's one