All Metro-style apps that will be installed on Windows 8 will come through the Windows Store. Microsoft is not allowing users to install apps from other sources, such as the web, so that it can ensure that Metro-style apps are reliable, secure and perform well. Here are some relevant rules for Metro-style apps.
App types. Windows Store will support free, paid, and trial versions of paid apps. Trial versions can be time- or functionality-limited but must provide a reasonable approximation of the full app.
In-app. Apps can provide in-app purchases, which are optional paid features, as well as advertising that conforms to Microsoft’s standards. Apps cannot simply be ads, and they cannot be a shell to a web site.
5 PCs and devices. Apps purchased from Windows Store must be licensed to run on 5 PCs and devices that are tied to a single Microsoft account.
No adult titles. No Apps with a rating over ESRB MATURE (or equivalent) are allowed.
One tile on the Start screen. If you’ve installed Office or Visual Studio on Windows 8, you know that they spews tiles all over your Start screen. Metro-style apps can install just one tile.
Privacy, security, reliability and performance. Metro-style apps must conform with strict guidelines regarding user privacy, security, reliability, and performance. For example, apps must perform the same on any PC/device type on supported platforms, must start up in 5 seconds or less, and must resume in 2 seconds or less.
In use, Windows Store utilizes a standard, Metro-style design, with a horizontal scrolling layout instead of a document-like vertical scrolling layout.
The Windows Store home screen is divided into groups, and it supports semantic zoom for quickly moving from one end of the UI to the other.