From the website of the British Society of Dowsers:
To dowse is to search, with the aid of simple hand held tools or instruments, for that which is otherwise hidden from view or knowledge. It can be applied to searches for a great number of artefacts and entities. It is most commonly known by most people in association with searching for underground water; not surprising considering the absolute need for water by man and his animals and cultivated plants which sustain him.
What is less readily known is that dowsing can be also used for searching for other underground features such as archaeological remains, cavities and tunnels, oil, veins of mineral ore, underground building services, missing items and occasionally missing persons.
Although no thorough scientific explanations for dowsing has yet been found it is frequently acknowledged that there is some correlation between the dowsing reaction and changes in magnetic flux when dowsing on site.
What is more difficult for the newcomer to accept is that dowsing can be carried out at a distance and, moreover, the distance itself has no bearing on the results; dowsing can be carried out for something in the next room or the next continent. This is of immense practical use for site dowsers who save themselves and their clients valuable time by initially, at least, dowsing at a distance to seek the direction of the nearest source, for example, or actually dowsing over a map of an area to determine more precisely the target of the search.
This particular faculty is frequently used by those practitioners using dowsing in the area of health when they are able to dowse for causative factors and suitable remedies at a distance from the patient, employing a sample or witness of the person, for example, on which to focus their attention.
Dowsing has been defined by Major-General Jim Scott-Eliot, a Past President of the Society, in his book 'Dowsing - One Man's Way as: 'The ability to use a Natural Sensitivity which enables us to know things we cannot know by the use of the day to day brain or by learning, by experience, or by the use of the five physical senses.'
You will find a wealth of information about dowsing, including history and uses, on the web site of the British Society of Dowsers.