The Communities feature of CASK (http://www.CodePlex.com/CaskDotNet) are beyond the scope of any single blog however I'll try my best to summarize it in a manner where you can appreciate the power.

First and foremost it helps to have a good hosting company (I've been with CrystalTech.com for years and have not been disappointed).  CASK works particularly well with host that support Domain Pointers.  I point all of my Domain Names to the same DNS and CASK takes care of the rest.  It parses the incoming Domain Name (below you'll see it know's I'm in as http://www.aclamplight.com/) and loads that site/community.  If it doesn't find it configured in the "Communities" list then it starts a Wizard to add it (more on the Wizard HERE).  I have literally put new sites up within 30 minutes (creating new templates is another story).

Note above that I am FTP'd into my primary server.  Under the server I have a Web folder which holds this databases domains.  I have another database, and different folder, that holds many more domains. 

If you create a folder with the "Community Name" under the Communities folder than that folder, and it's child folders, will have it's webcontrols used -- instead of the Common\Themes\<selected theme> folders.

This is particularly powerful if you want to share the themes between numerous client sites but do not want them to see "other client sites".  Note above that you see all of the themes under Common\Themes however since this is ACLamplight they only see their themes, not SingLikeTheKing or OsbornOrthodontic themes.  Recap: theme folders under communities will only be seen by the applicable domain where all sites will see themes under the Common\Themes.

It is important to note that the Communities\Common\Themes default folder contains ALL of the webcontrols that drive the CASK system screens (there are hundreds). The global themes (Arc, ArcMarble, etc) and client themes (ACLampLight and SingLikeTheKing) child folders only have to keep WebControls that they want to alter.  This is an important feature!!   You can start a new theme with empty folders (as I did with the Simple Template).  You then bring over the webcontrols (copy/paste) that you want to alter for that particular theme. The exception to this rule is the Styles.css, you'll want to copy a style sheet from one of the template that has matches your theme (and then alter it); when you see the number of style definitions you'll understand why I advise this. The "master page", or default page, for every them (global or client) resides in it's respective Skins\PageSkins\Default.ascx file.   This is the template that drives the layout of your theme.

Below you'll find the default.ascx page layout for the http://www.ACLamplight.com website; the controls are literally drag'n drop in their respective table sections.