The legal definition of Skim-Milk Cheeses varies wildly.
In general, it can just mean that some fat has been removed from the milk, though you may not know if all fat or even how much was removed. It can also mean whole milk has been used, with some skim milk added, or it can mean skim milk with cream or whole milk added.
In some countries, the legal definition may vary even from area to area. For instance, in New Brunswick, Canada, the definition is cheese made with milk that has less than 48% milk fat in it; in Manitoba, Canada, it’s cheese made with milk that has a maximum of 7% milk fat in it.
Just because a cheese is made from skim-milk doesn’t necessarily mean it’s lower in fat than other food products. You really need to read the label to see how many actual grams of fat there are per oz / 100 g. Dieters tend to prefer ones with 2 to 6 grams (preferably around 3 g) of fat per oz / 30 g,
Some traditional skim milk cheeses such as cottage cheese, ricotta and quark, are made as fresh cheeses (but note that quark can also be made from whole milk.)
Most, though, are made as aged cheeses, and tender to be harder and grainier than whole milk cheeses. They are harder because the fat that would have made them soft has been removed from the milk. The grainy characteristic also comes from the lower fat content. In whole milk cheeses, fat balances proteins in emulsification. But when there’s not enough fat to provide this balance, the proteins form grainy deposits. When this is not desirable, industrial food processors can overcome this without increasing the fat by using emulsifying salts which expand the surface area of what fat there is, increasing the emulsification that happens. Or they may use another, cheaper fat such as vegetable oils rather than milk fat. But if there are a lot of gums, stabilizers or other emulsifying agents added to a commercial skim-milk cheese, you may not get what you expect when it has melted.
One of the most famous skim milk cheeses is Parmesan.
Some people feel that when you take cheeses traditionally made from whole milk, such as cheddar, and try to make them from skim milk instead, that the cheese ends up with all the appeal of recycled sawdust, as the cheese was never designed for that kind of milk.
Skim milk cheeses can take longer to melt than full-fat cheeses.
Skim milk cheeses tend to have a longer shelf life.
In the past, Skim Milk Cheeses were just for the poor.
Farmers would skim their milk and either use the cream for butter or sell it. It made more economic sense to transport cream to a market than whole milk, because a pail of cream was worth far more than a pail of milk.
The skim milk left behind was often just fed to the pigs.
In the UK, the generic term for a Skim-Milk cheese was “Flet.”