Magento is a flexible, stable, scalable platform for e-commerce.
Magento comes in 3 flavours, Enterprise is the paid version (expensive) that comes with support and indemnity insurance from Magento, Community Edition is the open source version free to download with 99% of the features of Enterprise, and Go which is the smaller version and least customisable.
Most developers will use the Community Edition as it is free and open source but has a steep learning curve, particularly as there is no support. Magento will not function optimally in shared hosting, as it needs servers to be set up correctly to support it, otherwise it will operate slowly compromising business profits.
There are many downloadable extensions for Magento that once installed, add extra features to the already powerful standard set.