alright barney. basically, advent children is like final fantasy seven part2, a continuation of the storyline with the original characters
as for ffVII - SHINRA is a giant corporation that pretty much controls the world. They provide energy, are the government and the army. Basically, the world of FFVII belongs to SHINRA. they use MAKO to power cities. MAKO comes from/is lifestream (its been a while for me too :-P ) Now, there is a special combat group run by shinra called SOLDIER. Cloud thinks he was a member of soldier - that he was a partner to SEPHIROTH. However, he never made it to soldier, because he is just one of the "failed" clones of sephiroth. Sephiroth is an ancient - a CETRA (like aeris) and they are the true inhabitors of the planet. however, they are dying out, and HOJO wish's to clone them so as to enable the discovery of the PROMISED LAND - a place abundant in MAKO. JENOVA is also an ancient, and is sephiroths "mother".
The promised land is a place of supreme happiness, a place where the ancient can hear the voices of the planet (lifestream) clearly and live in peace. SEPHIROTH wants to absorb all the lifestream in the world by injuring the planet with METEOR and thus become immmortal and all powerful. HOJO wants to capture Aeris and clone her - he once experimented on her mother. INVOLVED HUH?
i think im a little fuzzy on the details, havnt played to game for so long. if im wrong, please sumbody correct me. any more questions, email me. mitchellmeek@hotmail.com
peace out people!

...im never gonna be good enough for you...