Still at Uni, but here goes:
PHYSICAL FILESYSTEM CODE : The code (almost a BIOS) that is used to write the data to the hard disk, i.e. the assembly code that is required to write data in the proper format for the file system
FILE ORIENTED SYSTEM CALL : A call made by a program to write to disk, i.e. ifstream File; File << "text";
STRUCTURE INDEPENDANT MANIPULATIONS : Functions (I/O, I presume) that are called the same independent of the file system, i.e. File << "text"; on ReiserFS, ext2, ext3, etc.. You don't need to know the file system structure
BUFFER CACHE : Just a temporary place to put the data while waiting for other things to happed, like a previous writing to hard disk call.
Plz someone correect me if I'm wrong, still studying the computer basic operations (assembly and so forth) maybe someone knows better.