The expansion packs add to the base game so that it becomes one big game. You play off of the most current game disk, whichever one you own. For example, I own all of the expansion packs and stuff packs, so I play off of my Late Night disk, which is the most current that's been released (at least until Outdoor Living comes out this week). You'll get the features of all expansion packs in the base game towns (Sunset Valley and Riverview). You sometimes have to place the new buildings in the older towns manually, but we have threads here to help you do that.
Each expansion pack has added a new town to play. When you start a brand new game, you select which town you want. The base game comes with Sunset Valley, along with a free download of Riverview from the Sims 3 Store. World Adventures didn't add a new home town, but you have 3 vacation locations you can visit. Ambitions added Twinbrook, and Late Night added Bridgeport. There's also an EA created town called Barnacle Bay that's available for purchase at the Sims 3 Store. As the expansion packs add up, you'll end up with several options for your home town and you'll also have each pack's new features unless you uninstall a particular expansion pack.
I hope this helps answer your question. Let us know if we can help with anything else.
