Shape: a line, usually enclosed so as to have height, width, and/or depth (i.e. a cube). A shape can be two- or three-dimensional and positive or negative.
Shapes can be divided into two categories:
1. Geometric: shapes created by mathematical laws. Examples are: triangle, circle, square, rhombus, trapezoid, pentagon, etc. (May include composite shapes - see below)
2. Free-form: irregular and uneven shapes which may include such things as puddles, clouds, and stones. Also included under the category of free-form shapes are organic shapes. Organic means "from living organisms" such as trees, flowers, animals, and even humans. (May include composite shapes - see below)
As the above definition states, a shape does not necessarily have to be an enclosed line. When we see a line, our mind can automatically enclose it to form a specific shape. For instance, what shape does your mind first envision when it sees four dots in a pattern from a die? Our mind will most likely join the dots to make a square, but they could very well be connected to form a completely different shape, such as a four petal flower.
Our mind likes to make connections. When it sees a drawing with sections missing (i.e.a dot-to-dot puzzle), it wants the lines to meet. Notice how, even though the lines do not connect, your mind can actually interpret what these shapes represent.
Positive Shapes: the shapes we have talked about so far have been created by space taken up by the object itself.
Negative Shapes: are made up of what's left in the picture format around the positive shape.
Look at this example:
What do you see?
If you see the white area as the positive shape, than it is a candlestick or vase and the black area is the surrounding shape is the negative shape.
If you see the black area as the positive shape, than you see two people facing each other and the white area is the negative shape.
When you draw, it is important to keep the negative shapes in mind as much as the positive shapes. In the illustration below, notice how the positive shapes blend in to become the negative shapes, and vice versa. Is this a picture of birds flying to the left or the right?
MC Escher: http://howtodraweasy.com/category/how-to-draw-a-easy/
Composite shape: A composite shape is created by combining two or more shapes to make a single shape.
Next, we see two organic shapes -- a hunter and a dog. Separately, it is easy to see that they would be different objects. When combined into a single composite shape, they become something new: