Master pages provide the look and feel and standard behavior that you want for all of the pages in your site. Together with content pages, they produce output that combines the layout of the master page with content from the content page.
Because Windows SharePoint Services is built on top of Microsoft ASP.NET 2.0, it supports master pages for defining elements that are common to all pages. You can specify all of the shared elements of your site in the master page or pages, and add content page-specific elements to content pages.
Master pages are a feature of ASP.NET 2.0 and work the same way in Windows SharePoint Services as they do in ASP.NET 2.0. For more information about working with ASP.NET master pages, see ASP.NET Master Pages Overview.
Benefits of Master Pages
In earlier versions of Windows SharePoint Services and Microsoft Office SharePoint Portal Server, you could share a page definition across sites, but could not effectively share common elements between page definitions. As a result, you could not easily change the look and feel of a page across the entire site. Master pages provide you with this ability.
Using master pages gives you the following single-location ability for change:
A better editing experience for Windows SharePoint Services pages
Developers can better use their time and resources by making design changes in one place and propagating the changes to all pages that use the master page.
Users can edit master page elements for their site in one place. They also can go to one place to make changes, and do not have to customize all of the pages that use those common elements.
User interface reuse
Users can easily create a page with the Windows SharePoint Services look and feel. By referencing the Windows SharePoint Services master page, a page based on it gets the Windows SharePoint Services look and feel, and also picks up any updates to the master page.
More consistent pages and a better end-user experience
Because all of the pages associated with a Windows SharePoint Services master page have the same consistent look and feel, end users can browse from a core Windows SharePoint Services page to a page added by a third-party solution or to a page customized by the Web designer, and not notice a difference in the way the site looks or in how its controls work.