Dowsing is a relatively innocuous practice of purportedly finding water with forked sticks or rods of some description. Those who are proponents of the art may also use the system to find other items such as precious metals and minerals. Often the success of dowsers is corroborated simply by anecdotal stories, which while convincing should not be used as the basis of definitive proof of any phenomena. As has been mentioned by others the plural of anecdote is anecdotes, not data. No matter how reliable the source, personal accounts are subject to errors in the form of confirmation bias and retrofitting to name two. The best way to measure the efficacy of claims such as this is to run appropriate studies.
Unfortunately when carefully designed blinded studies are performed the practitioners of this art seldom produce results better than chance. Commonly the explanation of dowsing is attributed to the Ideomotor effect. This refers to small involuntary movements which are made by an individual that can then be built upon for more pronounced movements that appear even to the practitioner to have no attributable cause. This explanation is accepted by skeptics as well as some of those who consider there to be legitimate use for Dowsing. The rationale is that the dowsing process is simply a means of tapping into the unconscious knowledge of the Dowser to find the correct answers. The ideomotor effect can also be linked to other movement based unusual phenomena such as Ouija boards and automatic writing and these, while traditionally having a supernatural explanation, could also be said to be connecting us to our all powerful subconscious, and this would be equally unfalsifiable.
The existence of belief in such things as Dowsing is quite easy to understand, the experience of apparently being able to find such a valuable resource as water can be quite powerful, both on the practitioner and the client. Add to this the general ignorance concerning how common underground water is, especially in areas conducive to settlement by people and you have the framework for an enduring mythology, as is to be found.