In this blog post, Abhishek Sur talks about the Application Bar in WP7 and its components - ApplicationBar Buttons and ApplicationBar Menu.
Source: DOT NET TRICKS
If you are working with Windows Phone 7, the first thing that you should have noticed is the very own Application bar. Application Bar is present in most of the applications that you use in your Windows Phone 7. This is basically a standard Toolbar with a menu associated with it which allows you to enumerate the commonly used commands in a standard location. While creating your application, Microsoft strongly recommends you to add an application bar, to ensure the user have common behaviour for every application. You can think Application bar similar to TaskBar of windows.