This is generally a proprietary coffee that a few coffee shops have started to sell. The general consensus is that it is seriously under roasted coffee. The advertising hype that goes with it would agree with this assumption although the people selling this product will not say anything about how it is made. Because un/bairly roasted beans are extremely hard a special grinder is required to grind the coffee.
One of the common claims about white coffee is that it is higher in caffeine than normal coffee. Coffee loses caffeine by volume as it is roasted so it probably does have more caffeine by volume. By weight caffeine content actually goes up so the caffeine content by weight is probably lower. One theory that has been proposed (but once again this is a gauess and probably varies from shop to shop) is that the blend may be all or mostly robusta to boost teh caffeine.
In Australia a white coffee or a "flat white" can be any number of different espresso and milk of coffee and milk drinks.
In the US and UK you will from time to time hear coffee with milk or cream referred to as white coffee.
White coffee can also refer to a cup of instant with a dash of cold milk.
If you are interested in trying white coffee try a search on Google.