Office 2013 has its own app store, and there are a number of different apps available for the various Office applications. These apps extend Office 2013's native capabilities.
For example, there are Excel apps that add new types of graphs and charts. The Office app store includes package-tracking apps for Outlook and a calculator app for Word, just to name a few. Best of all, most of the apps are free.
This is not to be confused with the Windows Store, which also provides enterprise-class tools.