The SMB (Server Message Block) protocol, renamed at some point CIFS (Common Internet File System), is the protocol behind resource sharing and remote administration functionnalities in Windows systems. It is implemented in all Windows NT systems.
For a thorough explanation of the SMB/CIFS protocol, see the SMB chapter  of Christopher Hertel's book, Implementing CIFS.
SMB 2.0 is the new version of SMB implemented in Windows Vista. For more information about SMB 2.0, see the SMB 2.0 pages on the Ethereal Wiki.