You can extend Office 2013, Office 2010, and Office 2007 by using Office developer tools in Visual Studio. These tools include project templates to help you create Office solutions by using Visual Basic or Visual C#, and visual designers that help you create custom user interfaces for your Office solutions.
If you want to create solutions that deeply integrate with Office and leverage the power of the .NET Framework, an add-in or customization might be a great option. However, if you're looking for a new way for users to interact with Office customizations, apps for Office is a great place to start.
Office 2013 and Office 2010 solutions using Visual Studio 2013
The story of Office development
What are apps for Office?
Apps are web applications that extend what a document, email message, or meeting request can do in Office.
Run them in multiple environments like Office desktop applications, Office Online, and mobile browsers—in the cloud or on your site.