When in CAS, (Create A Sim), there is a button with the word, "More" on it basically in the middle of the bottom of the screen. If you click on that it brings up an option to save to file, shown by a file icon, or save to bin or export. If you save to bin your created Sims will be saved to CAS with their original information attached to them. If you somehow lose your Sims while playing, you can then go back into CAS, click on "Premade Sims", select people, and your family should be there, but as individuals, not a group. You can then select them and recreate your family.
If you just save them to library while playing, they should show up there with the information for them at that point including any funds they may have accrued. So if you want, you can evict them from their current home thereby stopping any aging or progress they would make while you play another family. Then, when you want to play them again, you just put them back into their home.
Also, if you look in your Sims3 folder in your pc, you'll see a file named Saved Sims. Any Sims you have saved to library will also be there.
I'm pretty paranoid about saving things as I've so often lost Sims, builds and stories, including failing a dynasty because of it. So I regularly ALSO save a copy of pertinent files to my desktop or to a flash drive just to make sure I have the things/people I love.