iOS's file system: where are those files that are synced with the iPad via iTune? photos, videos, etc Where are files that are synced with the iPad via iTune stored?

For example, if you sync some image files onto the iPad via iTunes, where are those image files in the iPad?
Generally, the files are stored inside the application sandbox.
The application sandbox is a folder that stores the App and its associated files.
Every App has it's own application sandbox.
If you use file share in iTunes, the files are stored in a /Documents/ folder inside the application sandbox.
It is a bit tricky to access files that are stored outside the applications sandbox. 
Apple has a good and short explanation about files in it's iOS Application Programming Guide.
If you want to access on images from the PhotoLibrary, have a look at UIImagePickerControler.