Final Fantasy VII
Voice Actor: N/A
Jenova is indicated to be an extraterrestrial alien composed of cells that carry its will and power, even when separated. It arrived on the planet Gaia approximately two thousand years prior to Final Fantasy VII on a Meteor which created the Great Northern Crater.
As it emerged from the crater, it started to destroy the Cetra using manipulation and mutation, but was eventually defeated by the few Cetra that remained through the power of Holy.
Excavated From A Two Thousand Year Slumber
During a routine excavation in search of information about the Ancients, Professor Gast found Jenova's remains and incorrectly identified her as a Cetra. This lead to numerous experiments being performed using its cells, one of which was Sephiroth.
Although Lucrecia Crescent is Sephiroth's birth mother, Jenova's cells were injected into Sephiroth while he was still inside the womb, so Jenova is essentially Sephiroth's mother too.
That Day Four Years Ago
After ShinRa had finished with their experiments as part of the Jenova Project, Jenova's remains were placed inside the Mako Reactor in Nibelheim, which came back to haunt ShinRa when Sephiroth was sent on a routine mission to investigate disturbances.
Realising that the monsters creating the incidents were infact failed copies of himself, Sephiroth locked himself in the ShinRa Mansion to research the Jenova Project, which culminated in Sephiroth ripping the head off of Jenova's body inside the reactor.
Rejuvenation Under Sephiroth's Control
Following the incident, ShinRa moved the remains of Jenova back to Midgar where they were placed inside Hojo's lab. However, by this time Sephiroth had managed to take over Jenova's mind and he used Jenova initially to kill President ShinRa and release Cloud from his cell.
Jenova then leads a trail for Cloud using various forms of itself (e.g. Jenova LIFE) with the ultimate goal of persuading Cloud to take part in the Reunion of Jenova cells; something which Hojo deemed would always happen if Jenova cells were separated.
Defeated But Not Destroyed
As Cloud presses towards Sephiroth's location within the Great Northern Crater, Jenova tries to stop him one last time in the form of Jenova SYNTHESIS. Jenova is defeated again though and Sephiroth's demise follows shortly after with the two them being sent into the Lifestream.
Jenova's cells continued to live on though, with some of them residing in Cloud, and when the Lifestream is used to try and stop Meteor, Jenova's cells are spread across the entire planet.
Final Fantasy VII: Advent Children
Voice Actor: N/A
Despite Jenova's plans failing in Final Fantasy VII, it was not defeated as its cells were merely spread around the planet. They generally infected people with lower immune systems and manifested in the form of an illness called Geostigma.
Some remains were also salvaged from the Great Northern Crater by the Turks, which the Silver Haired Men are trying to obtain without much luck.
When Kadaj finally manages to obtain the Jenova cells from Rufus Shinra he isn't overly sure what to do with them. However, after some direction Kadaj soaks himself in the cells and turns into Sephiroth.
Cloud defeats Sephiroth again though and Aerith uses Great Gospel to cure the Geostigma that people had been suffering from. Not all of the Geostigma sufferers were cured though.
Crisis Core: Final Fantasy VII
Voice Actor: N/A
The scene in Nibelheim is re-enacted during Crisis Core, as Sephiroth finds out the truth about his past via Genesis and goes to research about the Jenova Project in the ShinRa mansion.
After he emerges, he goes to the Mako Reactor to be reunited with his "mother" - Jenova, but Cloud manages to ruin the party and sends Sephiroth plunging into the depths carrying Jenova's head.