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A "hub" is a piece of software that routes chat and search requests/results and facilitates clients to connect each other. It's not called a server because it doesn't share any files. All file transfers are being made directly between clients, not through the hub.
One can find hubs to connect to through the various hublists available in the client. A list of alternative hublists can be found here.
Read this guide on how to set up your own hub.