You have to fight Jenova (in various different forms) at four points during the game. Details of all the forms are on this page, in order of when you meet them. I am in the process of capturing images of each incarnation of Jenova, so only three of the four are currently available.

JENOVA*BIRTH
Level: 25 ... HP: 4000 ... MP: 110
This is the first incarnation of Jenova, and is fought while Cloud and the rest of the party are onboard the Shinra boat bound for the Costa del Sol. In the battle, Jenova uses a number of attacks including her "Terror Laser", which damages all party members twice in a row. Earth, fire and cold magic is most effective against her in this instance.

JENOVA*LIFE
Level: 50 ... HP: 10000 ... MP: 300
You fight the second incarnation of Jenova just after Sephiroth kills Aeris in the Forgotten City. Jenova's most deadly attack in this battle is the "Aqualung", which you can learn if you have an Enemy Skill materia equipped. However, it does 1500+ HP to all your party members so make sure you can take it. Earth attacks work best against this version of Jenova. Her attacks this time are all water-based, so equipping materia or equipment which absorb water damage is very helpful.
  [ Note: there is a Water Ring item in the rocky area between the Sleeping Forest and the Forgotten City, which absorbs water damage when equipped. Finding and equipping this item before you fight Jenova*LIFE will let you absorb all the damage she inflicts with the Aqualung attack - be sure to find it if you don't think your characters will be strong enough to survive the Aqualung. ]
  [ Another note: Jenova's only line of dialogue in the game, "beacause you are a puppet" (spelling mistake intentional, it's wrong in the game), comes when the battle finishes and Jenova finishes Sephiroth's sentence for him. ]

JENOVA*DEATH
Level: 55 ... HP: 25000 ... MP: 800
The third version of Jenova appears in the Whirlwind Maze on the way to the Reunion. Sephiroth summons her to kill Cloud and the rest of the party, just after he kills some more of the black caped Sephiroth-clones. In this battle, Jenova's attacks are all fire-based - so equip items such as the Fire Armlet to absorb the damage she deals. Her attacks are fairly weak this time, and include "Red Light" and a spurt of fire bubbles. She will also cast Silence on your party members every so often.

JENOVA*SYNTHESIS
A: Level: ?? ... HP: 60000 ... MP: ???
B: Level: 61 ... HP: 10000 ... MP: 600
C: Level: 61 ... HP: 8000 ... MP: 600
This is the final incarnation of Jenova, and is fought when you reach the bottom of the Northern Crater. She is the game's second-from-last boss, Sephiroth (in his various forms) being the last. In this incarnation, Jenova has two tentacle-like arms which you must also destroy: they will become useless once you deal enough damage, but can regenerate. It's best to use Enemy Skill and Summon materia to deal damage, although high-level spells like Ultima and Comet2 are also very effective. When Jenova is almost dead, a message will appear and count down from 5. If you don't kill her before the countdown reaches zero, she will cast Ultima and the battle will end. If this happens, your party will not actaully die and will still be able to continue the game - however, you won't receive any EXP, AP or Gil from the battle.
  [ Note: although I have yet to confirm this, apparently the amount of time it takes you to arrive at and kill Jenova*Synthesis affects the number of parties you get to take into the final battle. ]

Credit: Most of the information on this page is taken from Kao Megura's Final Fantasy VII FAQ, which you can get from gamefaqs.com. Copyright 1997-1998 Kao Megura. The images were screencapped directly from the game (the PC version) by me. Please don't use them on other sites without my permission.