Illustration, by hand or on the computer, is the translation of form, space, and light into a picture. Do you remember your first black-and-white pencil sketches as a child? Childhood doodles are the essence of drawing, which is the attempt to capture exactly how things look in a simple, recognizable form.
In our first drawings, we often sketch the outline of an object: a house or flower, for example. But look closely at most objects in the world, and you'll see that there are no outlines that go all the way around an object. Instead, you'll see contours: lines and shapes formed by contrasts in texture or color on an object and by the light that falls on it.