Curved lines are soft and flowing, and aesthetically pleasing, and therefore interesting to the eye and to photographers. They are also more dynamic than straight lines and tend to produce interesting graduated shadows.in an image. The "S" curve is often quoted as ideal and can be seen as soft and sensual. But any gently curving line can be used to advantage in composition - it does not have to be "S"-shaped. The most important characteristic is that the curves are gentle and flowing and not interrupted by jarring angles or breaks. A flowing river is often used in this way to link foreground and background in an image.
Curved lines have the effect of slowing an image down to a gentle pace, perhaps because we associate a meandering path or lane with a relaxing walk undertaken when we have plenty of time. It suggest the passage of time and a tranquil atmosphere. Zig-zag lines can also be effective, particularly when the angles are not too large. However they do not add all the qualities of curving lines to an image.