Apps for Office
Visual Basic for Applications (VBA) macros
VBA code is often stored and distributed directly within a document. While this makes initial deployment and distribution easy, it also makes updating code in existing documents more difficult.
Managed add-ins built with Visual Studio (VSTO)
For developers who want to leverage more extensive customizability of Office, or who want to target Office 2007 or Office 2010 (apps are available starting in Office 2013), VSTO is the primary option.
Read Selecting an API or technology for developing solutions for Outlook 2013 for an in-depth look at your development choices.