Your App is installed on SD or Phone, depending on user Settings on the Phone. You can't normally access those files eg. by browsing files while Phone is connected to computer - the apps are Isolated.
Depending on your App:
- if it's 8.1 Silverlight then you will find all the information here at MSDN.
- if it's 8.1 RunTime then take a look here.
Although you can't access files inside your App from outside the App, from WP8.1 you have access (read/write) to SD Card and other KnownFolders.
EDIT - you can also access your files by using Uri schemes.