Samba is a free software re-implementation of SMB/CIFS networking protocol, released under the GNU General Public License. The name Samba comes from inserting two vowels into the name of the standard protocol used by the Microsoft Windows network file system, "SMB" (Server Message Block).
See http://en.wikipedia.org/wiki/Samba(software) for more details.
Sharing a Windows hard drive from a Linux box is described in HOWTO setup samba (Windows steps to do this).
Also of interest is Basic Samba Setup for a Fileserver with Nightly Backups.