Lifehacker.com has a comparison  of 'media centers' as well as a good definition to use.  Sure there are some missing from the comparison, but generally those are the DIY solutions and not recommended or supported by systems intergators like Red Atom Networks.  The only one missing that is recommended is from ZeeVee- the Zinc  browser.  

These are examples of the new kind of media center for the home- those that largely integrate online and on-demand content with any of your existing 'personal' content.  We as system integrators are increasingly becoming advisors for content and media aggregation in addition to being experts in content distribution.  Red Atom Networks is here to help.  Give us a call.