There is never a real explanation during the game as to what exactly Jenova IS - Squaresoft never enlighten us as to the details of her race or origins. That means we can only speculate about these details, so there are a number of differing theories about what she really is.

The most common view of precisely what Jenova is goes like this: she is an alien who fell to the Planet from space, and tried to take over the world (and find the Promised Land the Cetra were searching for) by gaining control of the Lifestream. She befriended the Cetra at first but then spread a virus which turned most of them into monsters, but a few survived and managed to seal her away within the Planet. Two thousand years later, Professors Gast and Hojo find Jenova's body and mistakenly believe it to be that of a true Cetra. Fortunately for her, this meant that she was released from the geological stratum in which the Cetra had sealed her. That's all we can really gather from the game itself, so most of the other views on this subject are purely speculative.

During the game, you get a chance to see what Ifalna (Professor Gast's wife, Aeris' mother, and the last remaining true Cetra) has to say about Jenova. Her story describes what happened when Jenova came to the Planet, and the various events that followed. Click here to read it. Since Ifalna is the only character who truly knows what happened, it is safe to assume that her story is at least mostly true.

The most popular theory as to Jenova's being, and the one that ties in the best with Ifalna's story, is that she was an evil alien from another planet. Her reason for being was to travel the universe, taking over planets' Lifestreams. Either that, or she just had to take over planets by any means necessary in order to survive - in which case, taking over the Lifestream of the FF7 Planet was just a necessary part of the process. Finding the Promised Land would be an added perk, since it was rumoured to be full of Mako energy.

The Black Materia and the summoning of Meteor is an important factor in Jenova's reasons for being. Casting Meteor on a planet would weaken that planet greatly, making it much easier for her to take over - a form of head start, if you like. Assuming Jenova was in posession of the Black Materia from the start, it's much easier to understand how the Cetra came to get hold of it:

After successfully managing to seal Jenova away within the Planet, the Cetra had to come up with a way of making sure she (or another of her species, or even something of a different species altogether) could never succeed in taking over the Planet. They achieved this by trapping the Black Materia within the Temple of the Ancients, so it would be virtually impossible for it to be used again. (Remember that the Temple IS the Black Materia - the Materia is stored at the very centre of the Temple, and it is impossible to remove it without causing the Temple to collapse. In the game, Cait Sith sacrifices himself to get the Black Materia out for the rest of the party).

Sephiroth comes along two thousand years later and tries to do what Jenova did in the first place - to use the Black Materia to cast Meteor, weakening the Planet and allowing him to take over. It may or may not be the Jenova cells inside him that makes him do this, but that's not really relevant here.

Whatever the truth about why Jenova came to the Planet, the basic fact is that she was an alien with the rather useful ability to shapeshift. She appeared in various forms to the Cetra in order to scare them (and later to gain their trust); and appears in yet more different forms during the game - both when you fight her and when you see her body in the Shinra building and the Nibelheim Reactor. Her ability to shapeshift explains why she looks different every time you see her during the game.

There's an interesting theory saying that Jenova wasn't evil at all, and needed to use the Planet's lifestream in order to survive. In this case, she fell to the Planet and encountered the Cetra (who thought she was evil - from their point of view I suppose she was, since allowing her to survive meant sacrificing themselves) trying to get rid of her, so she had to get rid of them first if she was to survive.

FINAL FANTASY IX SPOILERS AHEAD!

The theory that Jenova HAD to take over Planet's Lifestreams means that for both Jenova and the Cetra it was a case of "kill or be killed". Interestingly, it reminds me of Garland in Final Fantasy IX trying to assimilate Terra and Gaia in order to ensure the continued survival of Gaia. Although Garland seemed evil at first - he appeared to want to destroy Gaia - it turned out he was just trying to help his people survive, and so could be considered good instead. This mirrors the theory of Jenova having to destroy planets (and take their Lifestreams, assuming every planet has one) in order to survive. Looks like another exaple of Squaresoft shamelessly re-using old ideas in new games... =P Seriously though, what would have happened if Jenova was just sent to secure the Planet for her race, and lots of other Jenova-type beings came to the Planet? Now THAT could be interesting...

Tying in with Jenova's existence and whether or not she is evil are the two differing theories about the Northern Crater. The most common view is that the Crater was formed when Jenova fell to the planet from the sky (remember the Cetra's name for her, "crisis/calamity/disaster from the skies" - the name is different depending on who you listen to and which version of the game you play) and was so heavy that she created a massive crater when she landed. The other theory is an interesting one: could Jenova have summoned Meteor on the Planet before she landed? If so, this could have been what made the Northern Crater. This makes a lot of sense, as it would mean that the Planet would be greatly weakened (thanks to the wound caused by the Crater) and it would be more difficult for it to resist whatever it was that Jenova wanted to do to it. Take a look at the Northern Crater formation page for more in-depth information on this issue.

John Poucher sent this opinion:

I think that Jenova is an angel that got banished from heaven by God for some reason unknown to me, which involved one of her wings being destroyed (her punishment), which results in why jenova only has one full wing and a half on the other side, and it also gives it a good reason on why she wants to become a god (revenge).

Feel free to send me your opinions, and I'll add them to the site.