Wow! The original The Sims!
That was a fun nostalgia trip!
If you snoop around in the "Tech" section on the ol' Moon Sims site, you'll find some tutorials that show what a sim is made of and how it all works.
How to Make a Zip Archive for Moon Sims has quite a bit of information in that vein.
How to Make Skins for The Sims has some pretty good stuff, too.