Shapes are area's that are enclosed within the composition. Shapes can be created by a line that continues into it's self or just filling in an area of color, even using other shapes to create a void in the middle creates that shape of the void.
Shape and volume
A circle is a shape, just like a triangle and so on. However when you add a gradient to the shape you have developed the shape into a volume and created a more dimensional depth.
Rectilinear and Curvilinear Shapes
Rectilinear shapes are created using lines that are straight with angular corners while curvilinear shapes are comprised of waving lines and circles.