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A Hub is a central point where all your tests are sent. Each Selenium Grid consists of exactly one hub. The hub needs to be reachable from the respective clients (i.e. CI server, Developer machine etc.) The hub will connect one or more nodes that tests will be delegated to.
Nodes are different Selenium instances that will execute tests on individual computer systems. There can be many nodes in a grid. The machines which are nodes do not need to be the same platform or have the same browser selection as that of the hub or the other nodes. A node on Windows might have the capability of offering Internet Explorer as a browser option, whereas this wouldn’t be possible on Linux or Mac.