You have limited save slots. These slots save the entire town in its current state and can be overwritten by a new save if you choose. Save slots are available in the Main Menu just after entering the game.
When you first enter a town, you get to choose an Active Family. This is the family you play every time you enter that saved file from now on. You can choose a premade family from the town or the household panel or make your own.
The Household and Lot Panel is the same from saved town to saved town. If you create a family or copy a family into the panel, they will be in that panel in any save file no matter which save you enter. Also, putting a family in a house does not remove them from the household panel -- you place a copy into the game but the family does not disappear from the panel unless you choose to delete them from it. You can also play a family to a certain point and save a copy of them (and their progress) into the household panel. As always, they show up in the household panel for every save you have. The panel never changes, just the active save file changes.
I'm sure it works the same way for houses, though I didn't get to tinker with that.
You can change your Active Family by entering the Options Menu, choosing Edit Town, and clicking the button with two houses on it (one big, one little), which says "Change Active Household." Your previously active household will lose its promised wishes (the things you clicked on to Promise, not lifetime wish or general progress).
So, if you like to make stories or want your Sims at a certain point before putting them into the "main" game you're playing, this is what I suggest:
Create a Main save file.
Create a Temp save file.
Create and play families in Temp Save until you're happy with them, then copy them into the household panel and place a copy in Main Save. Rinse, repeat until all the families you want are set up the way you want them.
If you want those families to stay pristine without too many changes, do the following:
- Options Menu
- Game Options (the gears)
- Lifespan (short to epic)
- Sim Aging (on or off)
- Story Progression (do neighbor Sims change?)