it means they run so much data by using widgets and other features that rely heavily on internet use so that its mandatory that you have an unlimited data plan because without one, your bill for one month will be more then your mortgage...
A 'data' plan is usually for touch screens or 'smart phones' which can acess the internet via mobile browser. its just a plan you must upgrade to in order for your phone to work (gps navigation, mobile web, etc) its for all the fancy stuff on your phone