Why do we use templates?
(From the Dreamweaver Help information)
One of the most powerful uses of templates is the ability to update multiple pages at once. A document that is created from a template remains connected to that template (unless you detach the document later). You can modify a template and immediately update the design in all documents based on it.
How are templates used?
When a site is designed the first page that is created will be the template. Within the template page regions will be created that can be edited on pages created off of the template. These regions are called Editable Regions.
A template author designs a "fixed" page layout in a template. The author then creates regions in the template that are editable in documents based on that template; if the author does not define a region as editable, then template users cannot edit content in that area. Templates enable template authors to control which page elements template users--such as writers, graphic artists, or other web developers--can edit. There are several types of template regions the template author can include in a document.