In Grid we have a central hub and we can connect various machines to that hub.These machines are called nodes.We can run various testcases on parallely on those nodes.Hub has a port number.Nodes have port numbers as well.Selenium browser run on a particular port number assigned to it.

A single test case can be run paralley on different nodes on different web browsers.

Mulitple browsers can be run parallley on one node or serially on one node i.e one browser after the other.

On one pc we can have multiple nodes as well.Then each node can have multiple browsers running.

We can make a single pc hub as well and on the same pc we have have multiple nodes and on each node we can run mutliple test cases or run multiple browsers.