Apps for SharePoint 2013 extend the capabilities of SharePoint sites and can be hosted in different environments. There are three main hosting models - SharePoint hosted, which allows client-side coding but no server-side code or databases; provider hosted, which allows server-side coding and databases but requires authentication tokens; and auto hosted, which also allows server-side coding and databases with authentication tokens. Apps can be implemented as full pages, parts that render within an iframe, or custom user interface actions. The document then discusses the SharePoint app architecture, app packaging/deployment, accessing SharePoint services and components from apps, and tips for app development.