Skimming is the name provided to a plastering method where a wall is plastered with a layer of thin coat.
It is usually applied to an existing plaster to smooth the surface area. It’s an extremely difficult task and shouldn’t be attempted by an amateur DIYer as the wall or ceiling might end up looking worse than previously!
The white layer of lime which is used to rough cement is called a skim
So overall, skimming is a subset of plastering. They are both used to decorate structures and increase the durability of a wall, but skimming is done to update an old building whereas plastering is done to a new one.
Another difference between skim and plaster is that plaster surface areas are constantly rough whereas a skimmed surface area is smooth.