Difference between dentry and directory?

[Xiangfei Jia <xjianz@xxxxxxxxx>]

I'm reading the VFS implementation. The VFS has four primary object types: superblock, inode, dentry, 
file.
A book says that " a dentry represents a component in path, which might
include a regular file. In other words, a dentry is not the same as a
directory, but a directory is the same as a file.". I so confused about
the difference dentry and directory.  Can anyone explain more
about it?

Thanks!

Fei