Magento today is the largest eCommerce platform in the market today - with a 26% the market share (April 2014), it is now owned by PayPal! It is a high-end eCommerce platform used by from companies from Oddbins to Fred Perry's. The Community Edition is open source, the Enterprise Edition has a commercial license.
It is important to note however that this is a different proposition to the Drupal & Drupal Commerce combination - its a pureplay eCommerce solution, not as focussed on the CMS aspect (albeit the EE version is greatly improved vs CE), with a number of features differences compared to Drupal commerce - if you are unsure as to the platform you should be using, talk to us!
Magento's strengths lie in its focussed nature, its ability to be customised and integrated with other systems and depth of features. Its ability to scale and offer a multi-store solution is second to none. With the correct configuration and development it is also a particularly strong multi channel solution (think of integration to eBay and Amazon with one shared product catalog). Magento can be more complex to develop and deliver than some systems - however in day to day usage it has one of the versatile administration system around.
There are two mainstream flavours of Magento available today:
Magento Enterprise Edition (EE)
This is enterprise level variant of the platform, and is strongly geared towards larger projects (for example, it includes an annual license fee of $18,000 to cover direct business support), with a significant feature set. It allows for vast catalogs, the highest levels of performance, audience segmentation, private sales, specific mobile commerce and much much more. The Enterprise platform can be geared to handle 1000's of transactions per hour.
Magento Community Edition (CE)
Magento Community Edition is by far the most popular version (given the significant variance in costs you really should have a valid rationale, budget and intent for using EE over CE), It is open source powered and supported, has an amazingly strong catalog system and supports up to 6000 SKU's. Although it feature set is more limited than EE, it has a huge available library of both commercial and free extensions to add capabilities.
Both platforms can be themed, customised and integrated with other systems as needed.