A roof valley is defined as the area where two roof fields join together making an inward angle.
Roof valleys are a very common area of failure in a roof, in other words where a leak is most likely to occur. There are two main methods of valley installation used to combat valley leaks. Woven or laced valley and the more common cut (or California) valley. Both methods may or may not be used in conjunction with metal flashing. The style of installation chosen will likely depend on what is normal for the local climate but using metal flashing will typically result in a longer lasting roof.
It is important to take care of the valley(s) of your roof by making sure that branches, dirt, leaves, etc... are not allowed to collect. When this happens drainage is impeded and water can begin to back up under the shingles eventually causing a leak.