The simple line offers great variety. Lines have a position, a direction in space and a single dimension represented by their length.
Lines may run horizontally, vertically or on a diagonal. Lines may be straight, curved, dotted, wavy, thick or thin.
They subtly organize and direct the eye of the observer, leading him through the piece of art as if on a tour, giving direction and communicating movement.

Seven Elements of Design | eHow http://www.ehow.com/list_7676900_seven-elements-design.html#ixzz2UJtdFd5Z