let me put it briefly then.
Shin-Ra is a company which profits from the use of generators to
drain Spirit Energy out of the planet and convert it into a source
of power.
Cloud and Sephiroth are soldiers of Shin-Ra
company.
Sephiroth and Cloud were injected with cell samples from
Jenova, an extraterrestrial
lifeform that leaves its subjects stronger and more resilient to
physical attacks.
As what u would expect from a villian, Sephiroth did all the bad
things and of course, the hero Cloud defeated him.
two years later, a disease called "Geostigma" has spread throughout
the world.
Cloud finds himself confronted by a strange trio of silver-haired
men who are the physical manifestations of Sephiroth's will and
spiritual energy.
Their leader, Kadaj, would later transform into Sephiroth after
absorbing the contents of a case containing Jenova's remains.
and then it leads to the final showdown.
simple? but that's only a basic summary. the story is way more
complex. 
