SQL Server supports a variety of network transports and protocols. It's important to have a basic understanding of how these technologies work and how applications typically make use of them. Understanding how to use them in your own applications will help you better understand how they are employed by SQL Server.
From an architectural point of view, SQL Server is just another application on the network. When it communicates over the network with a client, it does so using network APIs just as any other application would. When a client connects to SQL Server over the network, it does so via the network stack, just as it would were it communicating with some other type of server.