Leslie,
Under the hood, DNN uses a PortalID (number) to identify portals. The URL is, for DNN, considered a PortalAlias which it crosswalks to the PortalID to identify which content to display for a given portal.
You can have multiple URLs pointing at the same DNN install with totally distinct domain names: www.myGreenPortal.Com, www.HerYellowDog.com, www.smithytools.org, etc, etc ... you just have to set up the virtual directories in IIS to point at the same DNN install for each of these, and when you create a new portal, enter the specific URL as the PortalAlias for this portal.
Additionally, you can have what are called child portals, which share a domain name but use separate virtual directories, and have different content (are just as distinct as any other two portals). So that could look like www.myGreenPortal.Com/olivegreen/ and www.myGreenPortal.Com/seagreen/ ... so now you have same domain name but totally different portals (of course, they can look as similar as you want etc).
And you can have subdomains so that www.myGreenPortal.Com and lime.myGreenPortal.Com.
This isn't exactly a how-to tutorial nor is it intended to be, just a description of the relationships between URLs and portals and DNN installs. By the way, all of the above could coexist on a single DNN install.
As far as skins go, yes, you can design separate skins for each portal to the point that they look totally unrrelated to each other. It is possible to allow administrators to upload skins if you want - it's a setting on the host side. I generally do not as though it isn't rocket science, skinning still does require enough proprietary knowledge of DNN skinning that it is not difficult to mess things up badly. As an example, when I was initially learning skinning, I succeeded in uploading a container which was broken so that I could not get at the menu for that module to change the container. My only recourse was to delete the page. Not a problem if you are testing on an expendable page.
Not to make it sound difficult; I just messed up. But I don't expect or allow any of my commercial client-base or hardly any in-house web user folk (at my day job) to ever do their own skinning. If you create a couple-three related skins, with 5-8 related containers, they can select a different skin for a given page, and can control look of content on the page by selecting different containers for modules. That's where they control the look - within the limits of the skins/containers provided to them. Plus whatever you allow in the editor provider ...