This is the entire class of equipment used to connect the tubing and casing to a pipeline or to surface oil-water-gas separation equipment. The wellhead technically is a flanged device that attaches to the casing as the casing stick out above the ground. There several pieces of equipment that make up a wellhead assembly that further allow the tubing to be placed in the well and the flow of oil/gas to be controlled via values and chokes at the surface. The Christmas tree is the term used to describe the entire assembly as it looks vaguely like a Christmas tree in shape. On land, the wellhead equipment is very simple and simple to operate. Offshore and underwater, the equipment is very sophisticated and very expensive. Companies that manufacture this type of equipment are Cameron Iron Works, Kvaerner, NationalOil, and Weatherford. There are many others as well.