Drupal Commerce at its core is an eCommerce framework. It provides the basic components you need to build an eCommerce website and allows you to reconfigure and extend them as necessary. It's "build to suit" eCommerce at its best, and we owe it all to the amazing changes wrought by Drupal 7 and its other major contributed modules.
If you're referring to the project, use the full name (Drupal Commerce) and link either to this website (http://www.drupalcommerce.org) or the project page on drupal.org (http://drupal.org/project/commerce). If you're referring to contributions to the project, like contributed modules, core patches, learning resources, or support givers, feel free to link to the source material and refer to the people behind the contributions as contributors to Drupal Commerce.
It's best not to use third party logos to visualize the project. For example, early in the project's history, some cool mock-ups were posted to Flickr that were not selected as official logos of the project but have been used around the web as such. It's best for the project to maintain consistent branding between this website and the marketing materials you create. As the branding of the project itself is updated, so will this page with the most up to date resources for you to use.
For logos, see below or click here: (http://www.drupalcommerce.org/drupal-commerce-logo)