You have the Open Source Community which made osCommerce as a shopping cart. Then more features and installs were provided by the community which made later versions of osCommerce. Then still more features are added at the .org site. Next, some groups realized that people kept wanting some core set of modules and features installed on their oscommerce sites. So you have CRE Loaded. (the Chain Reaction Edition of osCommerce "loaded" with many of the features and modules). Then the CRE project has a split and Zen Cart is born. From there, Zen Cart goes on its own path and CRE Loaded goes on its path (not totally different, but both are great and have their own style). You also have VirtueMart which is osCommerce cart combined with CMS like Joomla/Mambo.
I've installed osCommerce and CRE Loaded before but Zen Cart is still new to me. I like some of the Zen Cart's powered shopping cart and I just can't wait to play with it.