Hello Nganvo,

In that data_dir  path, odoo will create folders: addons, filestore and session

in filestore odoo server will create a new folder for each database,  and in database, 
in table ir_attachment will be stored path relative to /path/to/desired/Odoo/filestore/db_name ... to actual file...

This folder will contain all attachments (from backend, and also all website materials like pictures etc...)

Hope this helps..

p.s.

odoo user should have rw acces to that folder, but under any cicrcumstances should it be 777 , never ever do that! 640 is quite good, and everything shlud work with that permissions !

Thanks.