Below is a shape. It is a sphere: a shape which can be separated from the background. Without the background it would still be a shape.
Getting more complex, the background of the above sphere is a shape as well. The sphere, or any object as well, is known as "positive space". The background, the space surrounding the sphere or positive space, is known as "negative space".
If placed in a black and white world, with no value added, positive and negative space would easily be seen, as illustrated below. It also can create illusions.
Here are some more examples.
Some artists in the past, as well as present day artists, use shapes to create new and exciting work. Their primary concern is shape and how these shapes can interact with each other. They care little of what the shape may represent in nature, but more on the nature of the shape. These artists are known as abstract artists. When viewing the work, try not to associate what an object represents in nature. The abstract artists use shape (or color) to produce an emotional impact on the viewer without the clutter of the visual world.
Click on the abstract work below and visit a website that displays some history and artwork of abstract art.
Mondrian's Grey Tree
Homepage
A Journey through History
Lines