There are just three maple tree species that are predominantly used to produce maple sugar because of the high sugar content in the sap of these species. This makes up just 2-5% of the maple tree population.
Maple sugar is what is left after you continue to boil maple sap after it has become maple syrup. Once all of the water has been boiled off, you are left with a solid sugar. This sugar is about 90% sucrose, the rest is made up of glucose and fructose.