Master Page

A master page is an ASP.NET page that has the file name extension .master. Master pages allow you to create a consistent appearance and layout for the pages in your site or application.

Master pages are a feature of ASP.NET 2.0 that work the same way in Windows SharePoint Services 3.0 and Office SharePoint Server 2007 as they do in ASP.NET 2.0. Using master pages, you can create a single page template and then use that template as the basis for multiple pages in an application, instead of having to build each new page from scratch.

Master pages, in order to render in the browser, actually require two separate parts, the master page itself and a content page. A master page defines the common layout and navigation, as well as the common default content, for all of the content pages that are attached to it. A content page is a unique page. When the page is rendered in the browser, the master page supplies the common content and the content page supplies the page-specific content.

First, you create a single master page to define both the look and feel and the standard behavior either for all of the pages in your site or for a specific group of those pages. Then you can create individual content pages that contain the unique content that you want to display on each individual page. The master page is merged with the content pages to produce a final, rendered page that combines the layout from the master page with the content from the content page.

Because there is full design-time support for master pages in Microsoft Office SharePoint Designer 2007, you can see exactly what your finished page will look like while you are in the process of designing it.

Imagine, for example, that you want every page in a site to use the same three-column layout, along with a standard header and navigation menu. In that case, you can create one master page that has the desired layout, and then attach all of the pages in the site to that master page. By creating a single master page, you avoid the work of re-creating the common content for each page. Also, if at any time you decide to change the layout of all of the pages, you need only modify the master page.

Every site in Windows SharePoint Services 3.0 and Office SharePoint Server 2007 that is created from a site definition includes a Default.master that defines the default appearance of the site. In addition, Office SharePoint Server 2007 also includes several custom master pages. These custom master pages, like the Default.master page, include the content placeholders that enable the display of SharePoint content such as lists and libraries and can be used with Office SharePoint Server 2007 to define the look and feel of an entire site. For more information on using master pages with Office SharePoint Server 2007, see the later section What is different about master pages in Office SharePoint Server 2007?

Ref: Master pages
http://office.microsoft.com/en-us/sharepointdesigner/CH100667701033.aspx