Ubercart is not a standalone application, but it's a module for Drupal. This means that you first have to install and customize Drupal, and then install Ubercart and all its related third-party modules. Imagine how powerful this combination is. You won't have just a selling point for your products, but a complete platform to interact with your clients and market your company.
Some key aspects of Ubercart that make it the number-one solution for online shops with Drupal are: