>About local drive and network drive, yes. But, even though I have heard the term many times, I don't understand "share". I will google for this and hopefully will find some explanations.

A network share is a windows resource that is marked as shared on a computer. It could be a folder, a printer, etc. A network drive is a share mapped to a letter on a workstation.
If you access a file through a share than WhoHas will see it. If you open the same file from the PC it resides on but as a local drive/folder, WhoHas will not see it as open because it's not done through the network services.