SMB stands for Server Message Block.
It is a protocol for sharing files, printers, serial ports, and communications abstractions such as named pipes and mail slots between computers and is being used in all Microsoft Windows versions for the so called "windows shares"
SMB can be used over TCP/IP, NetBEUI and IPX/SPX. If TCP/IP or NetBEUI are in use, the NetBIOS API is being used.
Developer Info on SMB on Microsoft FTP:
ftp://ftp.microsoft.com/developr/drg/CIFS/ ftp://ftp.microsoft.com/developr/drg/CIFS/SMB.TXT
Also see: Samba, a free Unix implementation of the Microsoft SMB protocol (access Windows shares from Linux / BSD).