Zen cart is an open source eCommerce solution that is built from the ground up by a group of very talented web developers, web designers, online store owners and marketing consultants that thought that eCommerce should and could be done differently and that's just wat they did. For free. The idea behind Zen cart is that online shopping cart solutions shouldn't be so hard to use for the end user. Zen cart is very easy to install even if you have little to none online knowledge. The amount of options Zen cart offers the store owners is tremendous compared to some other solutions out there.
It's considered to be really easy to integrate your own payment gateway to Zen cart, you can customize the entire site pretty easily and there are no specific hosting company requirements so in general - you can host your Zen cart website on any hosting company's server you want.