The Lifestream is under the surface of the Planet. That's where everyone's energy and memories go. It's also where the energy for new life comes from. If the Lifestream vanished, there would be no more life. The Planet would just fall apart, even!

And AVALANCHE... at first they were a terrorist group that fought Shinra. You see, Shinra was draining the Planet's energy to make electric energy at a profit... and other things. Well, AVALANCHE is still a rebel group, but now it's nine people. Barret and Tifa, Cloud and me, Vincent, Yuffie, Nanaki, Cid, Cait Sith...