Differences of WordPress com and WordPress org
WordPress.org is managed independently and freely modified, starting from the template, plug-in, plug the ad script, attach the javascript effects, to increase its own php functions. Need hosting and domain if you want to see on the internet. If you do not have a hosting and domain, you can be brain-tweaking on localhost.
WordPress.com has given wordpress.com and sub domain hosting services, such as yourname-.wordpress.com. WordPress.com is a free version and do not allow the user should to monetize it. But you can manage your certain themes widget and header.