In your C:\Program Files\Maxis\The Sims directory, or wherever you have it installed, you'll find a bunch of folders named "UserData" with or without a number after it. These are your neighborhoods.
UserData is neighborhood 1.
UserData2 is neighborhood 2.
UserData3 is neighborhood 3.
And so on. Back up these folders, as they're the main content.
Now you'll need to catch everything else that those neighborhoods use that don't come with the main installation. This includes any custom files you've added. If you're like me, you'll have all of your custom objects in the "Downloads" directory. Back this up, if there's nothing there.
All of your custom textures and such will be in the ..\The Sims\GameData directory. Be sure to get those too.
That should do it. Post if you've got anymore questions.