What is it?
Magento is a CMS and e-commerce platform written in PHP.
What is it like?
Magento can be a bit of a mixed bag - quite powerful in the hands of a very disciplined and experience team or horrible otherwise.
I worked with it for two projects and found it to be quite slow and unintuitive, especially when you had to traverse over a hundred SQL tables.
If I was given the choice on building an e-commerce based website, I would opt to go with an MVC framework.