In simple closed curves the shapes are closed by line-segments or by a curved line.
Triangle, quadrilateral, circle, etc., are examples of closed curves.
The shape which is not closed by line-segments or a curve is called an open curve.
A closed curve which does not cross itself is called a simple closed curve.
The curve which crosses itself is not a simple closed curve.
Here is a collection of closed curves. Except curve (d) and (e), the remaining shapes are simple closed curves.
There are three types of simple closed curves.
(i) Some simple closed curves are made of line-segments only. These curves are known as polygons. Triangle, quadrilateral, pentagon etc., are polygons.
(ii) Some simple closed curves are made by curved lines only. A circle is an example of such a closed curve.
(iii) Some simple closed curves are made of both line-segments and curved lines.
Related Concepts on Geometry - Simple Shapes & Circle