A Windows Phone application is made up of a frame and pages. The Pages are made up of XAML that define the UI and the Application Bar. The Application Bar is made up of Application Bar Icon Buttons and Menu Items.
MSDN: The default Application Bar is a row of icon buttons and an ellipsis along the bottom of the phone’s screen. The user can click the ellipsis to display the labels for the icon buttons and menu items if they are enabled. The Application Bar has built-in animation that is displayed as the menu is shown or hidden. Also, the Application Bar automatically adjusts when the phone changes orientation. When the phone is in landscape orientation, the Application Bar appears on the side of the screen vertically. This maximizes the remaining space for page content.
The Application Bar is a row of icon buttons along the bottom of the phone’s screen.