Jenova was an extraterrestrial being that crash landed on the planet leaving a huge scar on the earth that resulted in the Northern Crater. The Ancients, a group of prehistoric people (also known as the Cetra), called Jenova "The Calamity from the Sky". At first the foreign being appeared to the Cetra as a friendly visitor, but in reality it was bent on causing catastrophic destruction. Jenova spread a terrible disease to the Ancients, turning them into monsters and decimating their population and way of living. Eventually a small group of warriors fought Jenova and sealed her body away in order to ensure that her wickedness could never again emerge.
The Jenova Project
Eventually, scientists of the Shinra corporation unearthed Jenova's corpse from the icy rubble of the Northern hemisphere of Gaia. Professor Gast, the Shinra company's premiere scientist, mistakenly believed that Jenova was an Ancient and brought her to the Mako power facility in Nibelheim. There he and his apprentice, Professor Hojo, formulated the concepts and ideas that would eventually begin the Jenova Project. Gast believed that if he could infuse a combination of her cells and Mako, into the bloodstream of a human being, he could create a person with supernatural abilities and reflexes. The Shinra company leaped at the chance to fund this experiment as a way to create a new kind of super soldier. For their prelimary test, they decided to use a fetus as the prototype (the project was still too grounded in its developmental stages to risk sacrificing a grown human being). Therefore, Hojo offered the unborn child of his lover Lucretia (although the child is hinted to be in fact Vincent Valentine's) for the experiment. The test succeeded and resulted in a healthy baby boy who would later on be known as Sephiroth. He grew up to be the ultimate SOLDIER and became a hero for his exploits in the war with Wutai. However, he never knew anything about his parents or his true origin, only that the name of his mother was "Jenova".
After some time, Hojo continued the project and created a number of Sephiroth clones. He used the same process of injecting victims with Jenova's cells and showering them in a bath of Mako. Many of them became lame and retarded due to Mako poisoning, except for Zack Fair. These clones would eventually fail to serve their purpose seeing as how none of them succeeded in completing "reunion".
Jenova's Role in Final Fantasy VII
Chronologically, Jenova first appears in Final Fantasy 7 during a flashback sequence in Nibelheim. During one of his missions, Sephiroth travels to Nibelheim in order to investigate a mysterious occurence at the town's Mako Reactor. He eventually finds Jenova's body being stored in the Mako Reactor and goes insane. After researching the origins of the in the library of the Shinra basement, Sephiroth (believing he was an Ancient) goes on a mission to kill all the descendants of those he believes failed to protect his mother and the Cetra in their time of need. He burns down the town of Nibelheim and goes to the Mako
Reactor in order to free his mother. There, he decapitates Jenova and mortally wounds both Zack Faire and Tifa Lockheart before encountering Cloud Strife. After impaling him with his blade, Masamune, Sephiroth is shocked to see Cloud use the impaled blade as leverage to hurl him into depths of the lifestream.
The corpse of Jenova is then moved to Shinra Headquarters. During their mission to rescue Aeris Gainsborough from her Shinra captors, the party encounters Jenova for the first time. The headless body eventually escapes with the help of Sephiroth who in conjunction with his mother, wreaks havoc through Shinra headquarters and assassinates the company president.
During the ride on the cargo ship to Costa del Sol, the party encounters the shell of Sephiroth once again. Although the appearance is typical with that of Cloud's former role model, it is actually Jenova in disguise. The monster attacks the party as a being known as Jenova Birth. It appears paler than it does in its other incarnations and uses physical and laser attacks on the party.
After killing Aeris Gainsborough, the shell of Sephiroth once again turns into another form of Jenova. This time, the party fights Jenova Life in the remnants of a Cetra city. She appears more red than before and casts water based spells on the party.
The third incarnation of Jenova appears during the party's first voyage to the Northern Crater. After slaying all of the remaining clones, the Sephiroth shell transforms into Jenova Death. It apperas darker and more putrid looking and uses fire based spells on the party.
It is revealed during a cutscene that Sephiroth had been resting in a crystalline chamber inside the The North Crater all this time. He had been using his powers to control the different Jenova spawns while remaining stationary inside the Northern Crater with the original Jenova head he had decapitated in Nibelheim.
Eventually in the final disc, the party encounters Jenova's final form, Jenova Synthesis. It is implied that this is the form of Jenova is spawned from the original head that Sephiroth took during the Nibelheim sequence. Unlike the other forms, Jenova Synthesis appears more humanoid like (the other ones seemed more monstrous). In this form, Jenova appears to be a naked woman, breasts and all, with a large sac like pod protruding from her back. This time Jenova uses a variety of high level spells in combination with her devastating tentacle attacks. The strongest spells include Shadow Flare and Ultima(which it will caste after a five second countdown).
Jenova in Final Fantasy VII: Advent Children
Jenova is the key reason of strife in the CG movie Advent Children. The Turks salvage the remains of Jenova's head from the Northern Crater during the beginning of th e film (although they supposedly run into trouble from Yazoo, Loz and Kadaj). These remaining Sephiroth clones strive to find Jenova's Head in order to complete the process of reunion. It is revealed during the Bahamut fight sequence that Rufus had the head all along as he chucks it out of the window in order to infuriate Kadaj. Towards the movie's end, Kadaj melds with the Jenova remains in order to become Sephiroth. After Cloud lays it down old school, the final parts of Jenova are supposedly destroyed along with Sephiroth.