Apps for Office
Visual Basic for Applications (VBA) macros
VBA and the Visual Basic editor in Outlook provide the convenience to implement quick automation tasks for a small number of computers. They're also used in prototyping individual components of end-to-end solutions, which are then implemented and distributed as add-ins.
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.