This document discusses building Windows client applications for SharePoint 2013. It begins by explaining why apps and rich client applications are still important. It then covers developing apps for Windows 8, Windows Phone, and other mobile platforms. It emphasizes using REST/JSON to access SharePoint data from client applications and JavaScript frameworks like MVVM. The document demonstrates building a client-side app and recommends skills like JavaScript, REST, and HTML5 for mobile development. It concludes that hybrid apps allow code reuse across platforms and are a good choice for business applications in an increasingly mobile world.