In the Marvel Universe, Phoenix was a member of the X-Men from issues 101 to 137. Jean Grey/Marvel Girl, although no longer a member of the group, was involved in a conflict between the X-Men and the Sentinels, in a space station. Although the Sentinels were defeated, the X-Men had to fly back to earth through a radiation belt or storm. Although there was an isolation chamber, the cockpit was not protected and whoever flew the spacecraft would be exposed. Jean volunteered, since she could communicate with the astronaut in the isolation chamber via telepathy.
The spacecraft crashed into Jamaica Bay in New York. As the X-Men struggled to shore, Jean burst from the water, wearing a strange costume and claiming to be reborn as Phoenix. She was hospitalized and recovered in a short time. Alarmingly, it was discovered that her powers of telepathy and telekinesis had increased to unbelievable levels, greater even than Professor X. She had also acquired some new powers, such as flight, and the Phoenix force, a large hawk-like nimbus of fire which surrounded her when her powers were in use. Her powers were so great that she was able to contain the gravitational force of a neutron galaxy. Ironically, some of these new powers were a problem; for example, her telepathy was now so powerful that she couldn't turn it off; she automatically read the minds of people around her. Furthermore, the powers seemed to rise and fall in strength; often she would find her Phoenix powers would return to Marvel Girl levels at crucial moments (such as during a battle with Magneto).
Later, when undergoing tests with Moira MacTaggert in Scotland, Phoenix began having hallucinations of living in the 18th Century. It was later learned that Mastermind was bidding to join the Hellfire Club, and that recruiting Phoenix as the new Black Queen was his entrance exam. So, using the mindtap mechanism of the club's White Queen, he projected illusions directly into Phoenix's mind.
Phoenix did join the Hellfire Club, but broke out of Mastermind's control and defeated all the members. Unfortunately, all this turmoil did not help her already fragile mental state, and she metamorphosed into Dark Phoenix. She left earth, and returned only after devouring a solar system and angering the Shi'ar. Phoenix committed suicide on the moon, realizing that her powers would never fully be under her control and that her very existence posed a threat to the stability of the universe.
Later it was learned that Phoenix was not Jean Grey at all, but an alien being which had taken her form and left her body to heal in a pod under the waters of Jamaica Bay. This pod was discovered and Jean Grey returned to life, so to speak. Unfortunately her telepathy was lost or blocked, and many of the moments that her love interest, Cyclops, shared with her had actually been shared with Phoenix, so she had no memory of them.
There was also another character called Phoenix, Rachel Summers, who was from an alternate universe in which Phoenix had not committed suicide and had had Cyclops' daughter. In her universe the United States was under martial law and all mutants were hunted down by the Sentinels.