In addition to the standard shopping cart and real-time credit card billing that you'd expect in any ecommerce system, this module provides a customer service submodule (call center support), security, user tracking, gift certificates, mailing lists, an auditing system, order fulfillment, order tracking, special offers, and online reporting, among other features detailed in the overview below.
If you just want to sell a handful of items and not build a full online community, you're probably better off starting with ArsDigita Shoppe, downloadable from http://arsdigita.com/free-tools/shoppe.html. If what you want to sell is conference or course registration, we'll be adding those features to the ACS soon but meanwhile we do have the source code for register.photo.net. This code has been used to process thousands of course and marathon registrations.