I have heard the term Node in the Power Grid or Smart Grid, what does this node compose of? Is it a group of houses connected to the same substation?
"Node" (or: vertex) really is just a term from graph theory and describes any point in a graph where two edges connect.
Whether that is a single device, power outlet, house, a substation, a nuclear power plant or a national exchange where GWh are auctioned depends on the context. You'll need to really look into the details of each publication – "power grid" or "smart grid" doesn't define that.
A node is essentially a junction box provided to different regions of the state. It allows for more expansion from power grids to various houses in the region. A simple definition of a node is where two or more wires meet.
Some Nodes have smart systems that makes sure there is regulation in supplying mains voltage. Hope this helped.