Vertices : The corners of the polygons are called vertices.
Consecutive sides : Consecutive sides are those which have a vertex in common.
Diagonals : Diagonals are segments joining non-consecutive vertices.
In figure 3.2 A, B, C, D, E & F are vertices. AB has two consecutive sides BC and AF. Similarly two consecutive sides exist for the rest of the sides.
Segments joining A to all vertices except B & F are diagonals. Similarly, diagonals can be drawn from all the other vertices.