Microsoft today opened up its Office Store for developers and consumers. Office Apps are web based tools that will become part of Office and will enhance your experience.
Users will be able to install apps from Office Store as follows,
In the Insert tab of the ribbon choose “Apps for Office” and you’ll see apps from the Office Store. Users can learn more about apps in the store (and read reviews), acquire (or buy) them, and start using them straight away. All apps that are listed on the Store get extensively validated so users can trust them.
Developers can learn how to build an app on the new developer center (http://dev.office.com) and consumers can browse Office Apps catalog from the following link (http://officepreview.microsoft.com/store).
Read more about it in detail here.