In SharePoint 2013, you now have the ability to create apps for SharePoint. So what's an app? It's best described as a solution that carries a light footprint and uses standards-based technologies such as HTML5, JavaScript, and OAuth. Apps have a light footprint because they don't actually install on the host server, and that means they don't overload a SharePoint site with excessive API calls. Instead, you have a variety of hosting options for an app that gives you the flexibility to manage those services independent of the app package you add to your SharePoint site.
App’s Life Cycle for SharePoint development.
