Magento is an open-source ecommerce CMS (content management system) created the March 31, 2008. The name of the company who created it is Varien, building on components of the Zend Framework. 

Magento is an open-source CMS available under the Open Software License version 3.0. Since version 1.1.7 some parts are licensed under the Academic Free License version 3.0: it means you have to pay to use them but still can see everything about them. Most of the people think open-source means free. It is often the case but sometimes you have to pay to use an open-source software. Anyway, the costs are generally just to pay for the time of the developpers and not to make tons of benefits.

“Magento Enterprise Edition, a paid for version of Magento aimed at larger companies, was launched on April 15, 2009. This version provides a service agreement with Varien and starts at $11,125 USD per year.” says Wikipedia but I never had the opportunity to use it. Maybe you had? Let me know!