Electronic commerce, commonly known as e-commerce or eCommerce, consists of the buying and selling of products or services over electronic systems such as the Internet and other computer networks. The amount of trade conducted electronically has grown extraordinarily since the spread of the Internet. A wide variety of commerce is conducted in this way, spurring and drawing on innovations in electronic funds transfer, supply chain management, Internet marketing, online transaction processing, electronic data interchange (EDI), inventory management systems, and automated data collection systems. Modern electronic commerce typically uses the World Wide Web at least at some point in the transaction's lifecycle, although it can encompass a wider range of technologies such as e-mail as well.
http://en.wikipedia.org/wiki/Electronic_commerce

Shopping Cart Solution, modular templates, SEO optimization, MVC design, Automatic updating multiple currencies, languages, & Countries, Ajax AddToCart, product downloads, product options, multiple tax and weight classes

Axis is a powerful and free ecommerce engine. It has flexible modular structure and outstanding support for theme overriding and custom modules. A lot of great features comes out of the box.

Booot is a small, performance-optimized CMS with shopping cart functionality currently only available in Russian.

Magento is a new professional open-source eCommerce solution offering unprecedented flexibility and control.
Magento was designed with the notion that each eCommerce implementation has to be unique since no two businesses are alike. Magento's modular.

This is a application that allows you to run your own online marketplace or classifieds listing site similar to Craigslist. It was written in PHP and uses MySQL to for data storage.

The modified eCommerce Shopsoftware is an OpenSource shopsystem, published under GNU/GPL since the end of 2008.

NeoBill is a web-based Customer Management and Billing solution designed for web hosting providers, particularly hosting resellers. It is developed in PHP, uses MySQL as the backend database, and is licensed under the GPL.

Onxshop is not only great CMS offering integrated in-context editing and full design freedom without the constraints of limiting templates, but it's also stable ecommerce platform used in the current architecture in production environment since 2006.

OpenCart is an open source PHP-based online shopping cart system. A robust e-commerce solution for Internet merchants with the ability to create their own online business and participate in e-commerce at a minimal cost.

osCmax is a free, open source and very powerful e-commerce/shopping cart web application. It installs in minutes, and will run with very well even on small shared hosting accounts.