Master Pages play an important role in the sharepoint world similar to the human beings who plays an imporant role in the real world. Like every human being by default has 32 teeths and master page also has 32 different content tages. Sharepoint master pages are based on ASP.NET 2.0
Default Eight different master pages:
Placeholders ID Content
PlaceHolderPageTitle: The site title that is displayed in the browser title bar.
PlaceHolderAdditionalPageHead: Any additional content that must go in the HTML HEAD tag, such as style sheet references.
PlaceHolderGlobalNavigation: The bar at the very top of the page that contains the global breadcrumb trail, login controls, and the Help button.
PlaceHolderGlobalNavigationSiteMap: The breadcrumb trail within the bar at the very top of the page.
PlaceHolderSiteName: The site name that appears immediately below the top navigation bar.
PlaceHolderSearchArea: The search controls that appear to the right of the site name.
PlaceHolderTopNavBar: The navigation bar that appears immediately below the site name. This is known as the top navigation bar.
PlaceHolderHorizontalNav: The tabs within the top navigation bar.
WSSDesignConsole: The Page Editing toolbar that appears directly beneath the top navigation bar when you are editing a page.
SPNavigation: An empty placeholder immediately below the top navigation bar.
PlaceHolderPageImage: A page image below the top navigation bar, in the left margin of the page, and to the left of the page title.
PlaceHolderTitleLeftBorder: A border between the page image and the page title.
PlaceHolderTitleBreadcrumb: The breadcrumb trail that appears immediately above the page title.
PlaceHolderPageTitleInTitleArea: The page title.
PlaceHolderMiniConsole: Page-level commands, such as Edit Page on a wiki site.
PlaceHolderTitleRightMargin: A margin to the right of the page title block.
PlaceHolderTitleAreaSeparator: A separator between the page title block and the main page content.
PlaceHolderLeftNavBarDataSource: The data source for the navigation menu in the left navigation area.
PlaceHolderCalendarNavigator: A date picker for calendar lists. When the page displays a calendar list, the date picker appears at the top of the left navigation area.
PlaceHolderLeftNavBarTop: An empty placeholder at the top of the left navigation area.
PlaceHolderLeftNavBar: The navigation menu in the left navigation area.
PlaceHolderLeftActions: An empty placeholder below the navigation menu in the left navigation area.
PlaceHolderNavSpacer: A graphic that defines the width of the left navigation area.
PlaceHolderLeftNavBarBorder: The right border of the left navigation area.
PlaceHolderBodyLeftBorder: The left border of the main page content area.
PlaceHolderPageDescription: The text description for the page, underneath the page title.
PlaceHolderMain: The main page content.
PlaceHolderBodyRightMargin: A margin to the right of the main page content.
PlaceHolderFormDigest: The Form Digest time-out control.
PlaceHolderUtilityContent: Any content that must be added below the page content, such as footnotes.
PlaceHolderBodyAreaClass: Any content that must be added below the page content, such as footnotes.
PlaceHolderTitleAreaClass: Additional title styles for the page header.