You can add any static html you like into layout.master of your skin to show it on every page that uses the skin. You can enable per page skins in Site Settings and then be able to specify different skins on pages if you want to theme certain sections of your site differently. When a new page is created, by default it will inherit the same skin as its parent page unless its a root level page then it will use the site default skin. You can change the skin on any page though, its just a default.
If you want to use a content instance on all pages (that is all pages using a given skin) without having to add it to each page in content manager, you can do it using a ModuleWrapper control in your layout.master as documented here:
http://www.mojoportal.com/same-content-on-all-pages.aspx
Hope it helps,
Joe