@Goldenthrush All user data, everything you create or download into your game, is stored in Documents > Electronic Arts > The Sims 3. There are two types of custom content in Sims 3. One is .package-based files: these are installed into Mods > Packages just like mods are. If you didn't create the Mods and Packages folders and put any files inside, then you don't have to worry about this.
The other type of custom content is downloaded as .Sims3Packs and installed through the launcher. Before being installed, they're in the Downloads folder inside the Sims 3 folder, from where the launcher can read them. Once installed, the content is bundled into .dbc files in DCCache. (The files ending in .ebc are bundled store content, and the other files are temporary and can be deleted.) If the content includes a saved household or lot, it will also have a corresponding file in Library; a world will have a file in InstalledWorlds; a sim that shows up in CAS will have a file in SavedSims.
Additionally, there are extra files generated in DCBackup each time you install a Sims3Pack. These files are extraneous unless you want to upload content to the Exchange and want your cc to stick to the uploads. Otherwise, everything in DCBackup except for ccmerged.package can be deleted. The ccmerged file is necessary for premium store content to work.
