Master Pages in Sharepoint

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A master page in a publishing web site defines the outer frame of the web page. It contains the features, such as navigation links, that you want all pages in your site to share, and it provides a single place to control all of those features. A site uses a single master page, although large Internet sites might use more. For example, a corporate web site that is used to publicize more than one product could use separate master pages so that the content for each product is properly branded.
Types of Master Pages
There are two types of master pages: site master pages and system master pages. The site master page is used on published Web pages in your site. It is the master page that site users and visitors see when they view published content. The system master page supplies the layout of pages in the site that implement the user interface for commands ? for example, Document Library Settings. The system master page is also used in some team site templates ? for example, Wiki Site and Document Workspace.Master pages for all sites in a site collection are stored in the Master Page gallery in the top-level site in the site collection. Because the Master Page gallery is a SharePoint library, master pages have all the features of documents in Office SharePoint Server 2007, such as versioning, auditing, workflow, check-in and check-out, and content approval.