SHAPE

Shape is a flat two-dimensional area enclosed by line or the edge of opposing textures, values or colors. Shapes may be hard edged or soft and amorphic. Shapes become forms (three-dimensional) when their planar structure is defined.

Shapes fall into two categories of 1. Geometric: man-made shapes defined by mathematics (i.e. circles, squares, triangles, etc.) and 2. Organic: irregular or natural shapes (i.e. bones, leaves, clouds, etc.) 

Furthermore, shapes may be defined as being positive (the objects) or negative (the space around and within). In general, convex shapes are seen as positives and concave shapes are seen as negatives. As with everything concerning composition and image development, it is all about "relationships" and "associations" so one must look at the "whole" when trying to make sense of an image and spatial dimension.