I’m reading the documentation for Windows Phone development. Here’s what it says:

A set of tools will help the developer to submit and certify their applications for the Windows Phone Marketplace. Applications are submitted in a .XAP file format, which is essentially one compressed file that contains all the files that are needed by the application. Developers can track their submission status and then receive a notification once the certification is complete. After an application is certified, it can then be submitted for publishing on the Windows Phone Marketplace. Developers can set pricing and select the markets in which they wish to publish the application.

Application updates can go through the certification and publishing process again in order to fix bugs, add new functionality, or provide whole new versions.

Windows Phone Marketplace and Billing

The Windows Phone Marketplace provides the one place where developers can make their applications available for purchase by consumers. Both Mobile Operator and credit card billing are supported, making it as easy as possible for consumers to pay for the program.