App web Vs Host Web

The special website to which the app is deployed is called an app web. The website to which the app is installed is called the host web. Although the app web has its own isolated domain, it is in the same site collection as the host web.

SharePoint components are deployed to a special website with its own domain. This is called the app web.

• Deploying an app in isolated domain helps
• Blocking cross site scripting attacks
• Enforcing App permissions when App talks back to SharePoint using CSOM.

Please refer to know clearly about those:

http://msdn.microsoft.com/en-us/library/fp179925.aspx#TypesOfSPComponentsInApps