This question I'm like 99.9% sure was answered already but I never saw or paid attention to the answer, but I'd really like to know it now since it basically controls what I'm going to do with my sims in TS3.

Since I can't create new neighborhoods besides Sunset Valley and Riverview, what exactly does it mean that you can save them in 'slots'? I was hoping it meant it was exactly like TS1 and you went from 1 to 2 and 2 was a new, empty version of the same neighborhood so I could have several versions of Sunset Valley and Riverview with different people in each 'slot'. Is that how it works or am I wrong?

I think you are correct, but I'm having a little trouble understanding your wording.

My understanding is:
At any point you can save your neighborhood into a slot and restore it from that point, and you can have multiple saves.

For example you start the game and immediately create two saves, In save A you go in and kill sim #1 then resave. You can then go back to save B and have sim #1 live a long and happy life. You are effectively cloning the entire neighborhood.

I hope this is clear, its late here....

In a sense. When you begin the game, you have the option to save it -- think of it like a memory card with "savegame" slots in it. If you want to use a fresh version of the neighborhood, you can just start a new game and save it to another slot. Your hoods won't cross over, and your people will be different (although the "base sims" will be the same).

Edit: You can ALSO have copies of the sims you create -- once you make them, they stay in the simbin, and don't go anywhere. So if you make a family, they all kill themselves in a fire on the first day, and you want to play, all you have to do is drag the ones from the bin into a new, or the same, lot. By the same note, you can move them from the lot at any time into the simbin, and make a copy of them where they currently are in life. So if your single sim gets a family, you can copy the whole family, and play them from that period.

I see. Sorry about the wording, I have trouble explaining things >_<
For example if I wanted a neighborhood file with only anime sims and a neighborhood file with celebrity sims could that be done? Or will all my sims have to live in the same file? Because that would really stink :/

Yes you can have that. if you want.

They will be 2 different versions of the base town. You got it!

What do you mean two different versions?

You have one saved with your Anime version - no celebs

Your other version is the Celeb version.

2 distinct versions of the same neighbourhood the only difference will be the sims YOU have put in them.

Okay, that sounds like what I want! Thanks for all the help :P

You can get a fresh version of the town any time, but it won't be empty... it's based off the default template, so it will have Maxis premade sims and lots in it.

There's already a pre-emptied version for download at MATY -- it acts like a separate town, but it's Sunset Valley with all the houses and sims removed.

Or you can delete em yourself and just copy that save whenever you want an empty town to start with.