The custom contents come in simpacks or package files.
Some custom contents might crash the game or lag it to oblivion, therefore this mod is very useful. You can download it in 'modthesims' site, and the beauty of this mod is that it tells you which custom content has gone corrupt, which mods clash with each other, etc. Also, if you download custom content, I'd strongly recommend you one of the sims3 mods that is called 'dashboard'.
