The Design button lets you select the Page Type and Theme that the page you are on will use.  Each page type that is installed on your site is listed under the "Choose a Page Type" heading. You can see more page types by clicking the left and right arrows. Many page types will have icons associated with them. This helps to instantly see a basic diagram of the layout. However, this is not 100% reliable, so don't depend on it. Page type icons that look like Product Detail below are likely not representative of how the page would actually look.To use a page  type, simply click on it and then click save in the lower left. You can choose a theme for a page in a similar way. Navigate with the left and right arrows. Themes provide a snapshot of their theme as an icon, so if you see one you want to use, click on the icon and then click save. Your page design should now match that on the icon. If the theme is changed globally after choosing a different theme, then the theme for all pages will be reset to the newly chosen theme.

More information is available at http://www.concrete5.org/documentation/general-topics/how-a-page-looks/