What is Zen Cart? 

According to their site, "Zen Cart™ truly is the art of e-commerce; a free, user-friendly, open source shopping cart system." The software is being developed by group of like-minded shop owners, programmers, designers, and consultants that think e-commerce could be and should be done differently. Some "solutions" seem to be complicated programming exercises instead of responding to users' needs, Zen Cart™ puts the merchants and shoppers requirements first. Similarly, other programs are nearly impossible to install and use without an IT degree, Zen Cart™ can be installed and set-up by anyone with the most basic web site building and computer skills.

If you are familiar with osCommerce or CRE Loaded then you understand the basic workings of Zen Cart.  The osCommerce CRE Loaded 6.2 is more in line with Zen Cart then previous CRE Loaded versions, but they are still two distinct shopping carts.