It could be a lot of things. And we mean… a LOT of things! Mainly, an enterprise eCommerce must have the following set of features:
Enterprise Resource Planning
Customer Relationship Management (CRM)
Supply Chain Management
Manufacturing Resources Planning
Maintenance Management System
Enterprise Asset Management
3rd Party Integration
Reporting and Such features
There are a few frameworks that allow for this to happen. Those include Microsoft Commerce Server, which works very well with number of enterprise service bus and middleware including MS Biztalk. Then, there is Magento Enterprise and Apache Ofbiz.
These products can be leveraged to work on the solution needs and business handling at an enterprise scale.