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When two Windows for Workgroups nodes communicate, they must use a common set of commands. They can't just pass arbitrary strings of data around the network. With Windows for Workgroups and other derivatives of LAN Managers, a "dictionary" of network-I/O commands is provided by the X/Open-standard Server Message Blocks (SMBs). SMBs provide a well-defined method for servers and nodes (called consumers in the SMB definition) to communicate with each other.
The SMB specification is comprised of a list of network commands. For example, if a user wants to open a file that resides on a server, then the SMB command "SMBopen" is passed between the two systems. This occurs at the redirector level. The client PC's redirector sends the SMBopen command to the server PC's redirector module, which passes the request up to the server, which then interprets the request, and hopefully sends the file back to the client PC.
SMB commands exist for a wide range of functions, and provide a quick and cost-effective way to do the most common network functions. Commands exist for things like login security, printing, and working directly with files and directories. In other words, the SMB protocol is the heart of the network, providing a common language for all the clients and servers to use.
Note that by implementing SMBs on different systems, the client is able to communicate with them. If those systems were not running SMB-based servers, then the redirector would not be able to communicate with them. This is also very similar to the NFS model. As long as you've got an NFS server component on a system, then NFS clients can connect to it and exchange data.