Next I need to learn is to share folders in ubuntu such that I could copy/edit files from other PC over intranet. I tested this from my MacBook at home.
When I clicked sharing folders I have a choice of UNIX or Windows file system I picked Windows naturally. Samba, SMB, was installed automatically. I couldn't access shared folder from MacBook because I was asked to enter password which I wasn't asked to set when I shared the folder!?
Since SMB (which stands for Server Messsage Block) is a server for Linux therefore it has its own user and password system. The easiest way would be to edit smb.conf file under /etc/samba/. At around line 91 change 'security = user' to 'security = share'. Don't forget to remove ; (semi-colon) at the start of the line.
Of course it's better to have access control when sharing folders more reading to do here.