Microsoft has an updated model for building apps (now called “Add-ins”) for SharePoint and Office. These add-ins allow you to build extensions to these products and embed functionality into them through web based applications. Add-ins can be found in the Office 365 App Store in Excel, PowerPoint, Word, etc. and in SharePoint as well.
The framework is designed to work across Office Clients where Office Add-ins are supported including Windows desktop, web browser, Mac desktop and iOS on iPad. The framework also provides out of the box controls that you can use within your add-ins such as people picker, persona card, file picker, etc.
There are also user interface design guidelines available here for styling your add-ins.