SMB actually refers to a Server Message Block. It is used to share not just files over the network but also printers. SMB is not the primary network protocol of the Windows Operating System (TCP/IP is), but rather it is the primary transport mechanism of the Windows Network Specification. SMB is mostly transferred over NetBios or NetBeui (although older, more arcane implementations exist]. Under Windows, it is performed as SMB over NetBios over TCP/IP.

The source of confusion above may lie in the OSI modeling of SMB. I have seen numerous diagrams of the SMB protocol where it sits in the OSI Application and Presentation Layer, which sit on top of the Session Layer.