A node is a point in the circuit where two or more circuit elements join.
The current circuit has seven nodes (points a to g). Observing now the figure below, points b1 and b2 are considered to be the same point as node b. The same happens to points g1 and g2 which are considered to be the same point as node g. This is due to the fact that there is no circuit element between these points so they are not considered as separate nodes and are ignored.
Essential node is a node joining three or more elements.
On the above circuit there are 4 essential nodes (b,c,e,g).