When we study transverse waves which are transmitted along a rope we are seeing one-dimensional undulatory movements, which only propagate along the rope. 

However, light and sound are propagated in all directions from a single source. These are three-dimensional undulatory movements. In this case many points are subjected at the same time to the same vibration and constitute what we call a wavefront.

The easiest case to study is that of the waves produced in a pond by throwing a stone or stirring the water with a stick. This is what we investigate in the following visual, where you can choose between a point source and an extended source.