Magento is an open-source content management system for e-commerce web sites. The software was originally developed by Varien Inc., a US private company headquartered in Culver City, California, with assistance from volunteers.
Varien published the first general-availability release of the software on March 31, 2008, under the name Bento. Roy Rubin, former CEO of Varien, later sold a substantial share of the company to eBay, which is now the sole owner.
Magento employs the MySQL relational database management system, the PHP programming language, and elements of the Zend Framework. It applies the conventions of object-oriented programming and model-view-controller architecture. Magento also uses the entity–attribute–value model to store data.
Read more about Magento at Wikipedia.
I am using Magento since 2010. Besides installing and configuring it, I have experience in modifying code of Magento and it's extensions to achieve different behavior than default one.