While developing apps for SharePoint 2013, you might be wondering about the host web and app web terminologies. Let's talk about them in a clear and a precise manner in this blog.
Suppose, you are developing a SharePoint 2013 app for your organization. In that, you would require the App to access and to use the SharePoint components such as the lists, content types, workflows, and pages. In this Case, all your SharePoint components should be deployed in a separate SharePoint site, called as the App Web.
The Host Web is nothing but the SharePoint site where the App is actually installed. So, to conclude, all the resources accessed by a SharePoint web has to be deployed in a different site, named as the App web. And, the actual site where the app is deployed is called the Host Web.
Hope this helps!!