eCart 5 is an extesnion that can be used to add eCommerce functionality to your existing pages. You would use eCart, DataAssist and SecurityAssist at the least to crate a full On line store including product catalog and Back end administrative section to create your online Store.
PowerStore is a complete online store that has already been built using eCart, DataAssist, SecurityAssist and other tools. It is a site that already contains the Admin Back end that can be used to populate the online catalog.