VirtueMart is an Open Source E-Commerce solution (a Component or Plug-in) for the Joomla! Content Management System. It can be run as a Shopping Cart, or in Catalog mode.
What is Virtuemart?
VirtueMart is a complete eCommerce solution. It is released under the GNU General Public License.
VirtueMart is free software. No limitations exist for VirtueMart. You can create and maintain a site with an unlimited number of categories, products, orders, customers, etc. In addition, lots of extensions (both free and commercial) are available for Virtuemart. Most of them can be found in the Joomla Extension Directory (JED).
Who is behind Virtuemart?
VirtueMart currently has only a very small team. There are 3 to 5 active developers, who help evolve the Core System and update the Project's Open Program Sources. There are a lot of developers and active users on this forum who help pushing this Application forward into the right direction.
The first place to look for documentation on Virtuemart is in the Virtuemart Knowledgebase.
As of August 2014, Virtuemart is not compatible with Joomla 3.x, although the upcoming Virtuemart 3 is expected to work with Joomla 3.x (http://virtuemart.net/news/latest-news/460-updates-about-virtuemart-3-support-membership)