The Jamroom Skin defines the overall look and feel for ALL areas of your Jamroom EXCEPT the Jamroom Control Panel, and the individual Artist/Member Pages.  The skin controls the look of your main "index" page in Jamroom.  The available Jamroom Skins can be found in the jamroom/skins directory, and can be changed in the Jamroom Config -> Jamroom Skins section when logged in as the Master Admin:

 

The Artist/Member Profile Themes are the templates and Jamroom template functions and variables that make up the components for an Artist or Member "site".  It is often asked why the Artist/Member themes are NOT part of the Jamroom Skin - this is to allow an Artist to have a different "look and feel" to their Artist Page then you have for the overall Jamroom site.  Note that you can limit your Jamroom Artists and Members to only using 1 Artist Theme if you do not want your Artists and Members to be able to select their own Artist Theme.  This can be defined in the Jamroom Quota config:

 

Within the jamroom/styles directory you will find the CSS files that make up the different "Control Panel Styles" that can be used in your Jamroom.  If you want to force all of your Users to use the same style when logged in, make sure that only the style you want to use is in the jamroom/styles directory.