The Virtual File System (VFS) defines file system related sys calls such as open(), stat(), read(), write(), chmod() and so on (See fs/open.c).
Inode
An inode represents a file system object such as a regular file, directory, symbolic link, etc.
It is the central data structure used in the Virtual File System and contains information about the file: type, size, permissions, last modified time etc (see the iattr struct and the inode struct).
It should be noted that an inode represents a file, not a file name. A file (i.e. inode) might have many names (i.e. hard links).