Varien, the company that owns Magento, was working with osCommerce. They originaly planned on building a fork of osCommerce, later decided to rewritte it as Magento. Magento officily started to be developed in 2007 and its first beta version was release on August of the same year.
What is it
Magento is a e-commerce system developed by Magento INC, and it comes in three editions, being the CE edition the free one. Magento e-commerce is becoming more and more popular over the world, being used as the base for several big companies such as Fox, Nokia and others.
(Check out the list of companies that use Magento in this link: http://www.magentocommerce.com/showcase). This is due to great flexibility that it provides to the developers of online stores. Its structure is based in modules e was developed based on Zend Framework.
With over 150.000 stores over the world, present in more than 50 countryes, Magento is considered the most popular e-commerce system in the world. In a research by Aheadworks in Setptember 2012 it was stated that Magento is used by 31,4% of the companies. Not only that, the term "Magento" is already more searched online than "e-commerce", according to Google Trends.
Oficial Site: http://www.magentocommerce.com/