Is it possible to create event receiver or workflow on SharePoint-hosted apps in SharePoint 2013? I have not much knowledge about SharePoint apps. If yes then how to create it...
Can we use Remote Event Receiver in SharePoint hosted apps?
When you attempt to add a Remote Event Receiver to a SharePoint-hosted app project, Visual Studio will add a new web application project to the solution that includes a remote event receiver WCF service. Because a remote event receiver relies on a WCF endpoint to "remotely" handle the events being raised from SharePoint, and this WCF endpoint contains server-side code, by definition it cannot run in a SharePoint-hosted app. For more information, see this remote event receiver how-to on MSDN.
You can, however, create and deploy simple workflows within SharePoint-hosted apps. You can read more about workflow-powered apps for SharePoint here. In general, you should only use workflow in a SharePoint-hosted app when the workflow is integral to the app for SharePoint itself, as the workflow will only be operating on lists within the app web. (Note that in order to successfully build my SharePoint-hosted app project using the Microsoft Office Developer Tools for Visual Studio 2012 - Preview 2, I had to add references to System.Web 22.214.171.124 and System.Drawing 126.96.36.199.)