So far we've thought about waves as moving in one dimension, but in practice they usually move in two or three dimensions. To visualize this, we can color the crests as red and the troughs as blue. In two dimensions, the waves go out in all directions, and so all the crests of all the waves form a circle, which is called a wavefront. The distance from one wavefront to the next identical one is one wavelength $\lambda$. As time goes on, these wavefronts move away from the source with speed $v$.
