http://www.thesims3.com/exchange is the official website where user made content can be downloaded. It uses the Sims 3 launcher to download and install custom content. Both the exchange and Sims 3 launcher require the same account.
http://www.modthesims.info/index.php is a third-party host for mods and custom content.
More information about this mod at The Sims Wiki
HKEY_LOCAL_MACHINE\SOFTWARE\The Sims\The Sims 3