Used to be involved in this in NZ a few years ago at a provincial airport surrounded by a large amount of bush.
Baisc equipemnt consists of an underslung water carrier called a "monsoon bucket". The buckets has a series of holes in the side, each of which can be closed by a rubber seal. The number of holes closed will determine the capacity/weight of the bucket according to the underslung capability of the helicopter. The water is released by a cable attached to a valve at the bottom of the bucket.
Works well on localised fires, but probably of limited use against fires with a wide front...