"" searches in current file's path.
<> searches in global include paths.
Edit: You asked for absolute path and relative path.
Assume you have a file structure as follows:
folderX
-fileX.a
-fileX.b
-folderX.Y
-fileX.Y.a
-fileX.Y.b
-folderX.Z
-fileX.Z.a
Then, the absolute path of fileX.Z.a would be folderX/folderX.Z/fileX.Z.a, assuming that folderX is the topmost directory available. The relative path of fileX.Z.a relative to e.g. fileX.a is just the part folderX.Z/fileX.Z.a, i.e. you start the path in the directory where fileX.a lies.
