Along with the Farm solution and sand solution we are familiar with SharePoint 2013 another 3 mode of hosting for SharePoint 2013, this are app hosting model which will be running 100% out of the Farm in either in browser or in a webserver or in cloud.
App model are much cleaner simpler and upgrade process.
App Communicate with SharePoint via REST/OData/CSOM
App are granted permission to SharePoint via OAuth.
SharePoint hosted app – Run at browser implemented using JavaScript-client side app model.
Developer hosted app – Run in the web server or cloud, server side code. Using this model other language implementation says Eclipse or LAMP can be done.
Azure auto hosted app- similar to developer hosted app but create its own database and web application for each tenant in the multitenant mode.
