It is available on the App Web under
"The APPWEB URL"/Lists/"The list internal name"
But really you should access the list (CRUD items) via Client object model or REST.
The list is meant to be "hidden" to the users of the app, and it is upp to you to provide interfaces to work with items in the list
And just a clarification: You need to use SharePoint hosted app to be able to use SharePoint artifacts like Lists directly in the App project
Though this one tells another story:
While this below tells this story (MSDN):
Benefit: This approach enables you to reuse common SharePoint
artifacts, such as lists and Web Parts.
use any server-side code.
http://msdn.microsoft.com/en-us/library/fp179887.aspx describes different hosting alternatives and their ups and downs. (But it is surely not too extensive)
So, you could probably use a List also in the other hosting alternatives, but it will need some more work than just adding the item to the App project for you to be able to access it I guess