Most supermarket ham sold today, including premium ham, is formed or reformed ham. Formed ham is muscle meat from the leg bones. It is chopped and passed under needles which inject it with a solution of water, sugars, preservatives, flavourings and other additives, or put into a giant machine resembling a cement mixer and mixed with a similar solution. The process dissolves an amino acid called myosin so the meat becomes sticky and, when put into moulds, comes out looking like a whole piece of meat.

If the ham is to be presented as a traditional cut, a layer of fat is stuck round the edge of the mould to make it look as though it has been cut off a whole leg.

Reformed ham is made from chopped or emulsified meat which is not necessarily all muscle meat. Scraps left over from making formed ham may be used in reformed ham. - http://www.guardian.co.uk/uk/2005/nov/03/foodanddrink