Salt Pork is a slab of pork that is almost entirely fat. The fat is cured in salt, and not smoked.
It is not used as a "meat" in itself. It is used instead small chunks to provide flavour and mouth feel to recipes.
Salt Pork may come from the back, the belly or either side of the back of the pig. Occasionally it may have a streak or two of lean meat in it, particularly if it is cut from the belly of the pig.
Sometimes it may have rind with hairs on it.
Some people in North America say it's the same cut as American bacon (aka streaky bacon), but unsmoked. It certainly is that way in some places these days, but traditionally it wasn't at all. Traditionally, if there was a streak or two of any meat in it at all, you were lucky. It really was just all fat, and way cheaper than any kind of bacon. Now, though, with the increasing presence of meat in it, it is sometimes as expensive as bacon.
It is used a great deal in cooking in the American South, particularly in soups, bean dishes, and with cooked greens. In the American south, it was sometimes referred to as "white meat."
Not the same as "fatback."
Some advise to soak Salt Pork first before use to leach some of the salt out, but the saltiness really depends on who processed it.
Ham (though it is much leaner so the taste will be thinner and it will have a smoked taste that the recipe probably didn't anticipate); unsmoked streaky bacon (available in the UK), or smoked streaky bacon (aka "regular bacon" in North America) blanched to remove some of the smoky taste, or fatback, with additional salt added to the recipe to compensate for the lack of salt in the fatback.
Literature & Lore
"The best pieces of fat salt pork come from the back, on either side of backbone." -- Fannie Merritt Farmer. Chapter 16. Pork. The Boston Cooking-School Cook Book. Boston: Little, Brown. 1918.
"Meat" in Florida is one thing--white bacon. We call it white bacon to distinguish it from breakfast bacon, or side meat, and it is, simply, salt pork, or, to the army, sow belly. If it is under-rated in the north and by the military, it is perhaps over-rated in Florida, for it is the staple meat. Affluent rural families serve it three times a day, no matter what other meats may be on the table, poor families have it as often as they can afford it, and town families of rural antecedents serve it when the nostalgic hunger becomes too great... White bacon is cooked everywhere in about the same fashion. It is usually soaked a little while in warm water or in milk, squeezed dry, dipped in flour and fried to a crisp golden brown. The large amount of grease that fries from it is poured into a bowl and this to the backwoodsman is "gravy." It is solid grease, and it is poured over grits, over sweet potatoes, over corn-bread or soda biscuits, and how country stomachs survive ten hundred and ninety-five servings of this a year is a mystery past my solving." -- Rawlings, Marjorie Kinnan. Cross Creek. New York: Charles Scribner's Sons. 1942.
"When the peas are shelled you get a big saucepan and put in a piece of salt pork about the size of a package of cigarettes. But first take a knife and slice the pork down to the rind about every half inch so it will cook all the way through. Three or four slices of smoked or sugar cured bacon will do, but you won't get the flavor you get with a chunk of white salt meat. Put water in the pan, put it on the stove and start it cooking. -- Attributed to: W.D. Bedell, The Houston Post, 1961. In: Town Talk column, by Howard W. Rosser. Winnsboro, Texas, USA. 17 August 1961. Page 1.