By placing a design element on a master page and then assigning several pages to use that master page, you ensure that all the pages incorporate that element. Of course, each individual page can have its own independent 'foreground' elements.
To create a master page:
From the Pages panel, click Add Master on the Master Pages window.
From the dialog, choose whether the master page will be Single or Facing page (i.e. two-page spread). This is disabled on non facing page documents.
Choose settings from Dimensions or Margins tabs. These are mostly derived from Document Setup.
Click OK. The master page is added to the Master Pages window.
You can edit the master page by -clicking its thumbnail, and selecting Spread Properties. This lets you convert from single to facing pages and vice versa, as well as adjust general page properties.