MOSS Question:

Master pages provide a base look and feel and standard behaviors that are needed for all of the pages in your site. The content page combines with the layout of the master page to form the complete output. Microsoft SharePoint Foundation is built on top of Microsoft ASP.NET, therefore, it supports master pages for defining elements that are common to all pages.

Advantages of Master Pages:

People can save their time and resources by making design changes in one place they will be reflected in all pages that use the master page. The User interface can be reused.

More consistent pages and a better end-user experience