SMB was originally invented by IBM but the most popular version is modified heavily by Microsoft, and goes by the name CIFS - the 'Common Internet Filesystem.
SMB works through a client-server approach, where a client makes specific requests and the network server responds accordingly.
The SMB servers make their file systems and other resources available to clients on the network. Client computers may have their own hard disks, which are not publically shared, yet also want access to the shared file systems and printers on the server.
The Microsoft implementation of SMB is known as NetBIOS and clients may use any transport protocol (e.g. NetBEUI, or IPX), though TCP/IP is the most common. To this implementation, Microsoft has added several tweaks and incompatibilities that are not part of the original SMB protocol.
The Samba software is a free software reimplementation of the SMB protocol and the Microsoft extensions to it.