Regular polygon

A polygon which has all sides are congruent(equal) and all angles are congruent (equal) is known as a regular polygon.

If a triangle has all the sides are same and all the angles are same then it is a equilateral triangle or a regular triangle.

If a quadrilateral has all the sides are same and all the angles are same then it is a square or a regular quadrilateral.

Irregular polygon

A polygon which has all sides are not congruent(not equal) or different in length and all angles are not congruent (not equal) or different in measure the polygon is known as a irregular polygon.

Above all of shapes are polygons. We notice that all the shapes are drawn with only straight lines and all the sides are connect. 

Not a polygon

In above shapes we notice that one shape is curved and other shape all the straight lines are not fully connected, so it can’t be called a polygon.