As a very quick "snapshot" of the overview of Christianity as I believe it to be............
God made Earth habitable for man (that is a slight departure from God "created Earth" - but it is permissable to interpret the Bible in that way; Jayhawker Soule wrote a thread on it.
God rules the heavens, and he decided to give man a chance to rule and govern over Earth.
The trouble was that one of God's angels decided he'd quite like to be the boss (The Christians call him Satan), and Satan has been down here reaping havoc ever since.
We all have free will, and can either be tempted into following Satan (he's quite a trickster - he'll promise you anything to get you on his side), or we can choose to follow God's rules and try to be as good as we can be - Since I consider myself a heck of a sinner, and I
am trying, it isn't easy; still it is up to each one of us to take whichever line we wish to.
To those who have chosen to follow Satan, the end is "non-existance" - for those who can resist his ways, there is eternal life in Paradise; Paradise is whgere the Garden of Eden was before that got wrecked before Adam let God down, and allowed Satan to tempt him.
Still, God could just have come down, and booted a few where the sun doesn't shine. But he sent his son, Jesus Christ, to come down and teach us about all this.
God actually sacrificed his son Jesus Christ as a sacrifice - or a payment for - the ability for man to still be allowed into Paradise if he truly believs in Jesus Christ as our saviour - by his sacrifice, and as long as we try to be good (we never will be, because we are all imperfect), and we love Jesus Christ, we should get there in the end.
I guess that's the system as briefly as I can put it, and hopefully correctly - Please Christians, forgive me if I have got it wrong, and put me right.