There are two types of loads, typically, dead loads and live loads.
You break up the building into categories of components with the same weight metrics, for example: 6″ concrete slabs, 4″ steel beams, brick veneer, windows, etc, etc, — you may have different types of metrics so your concrete will be measured by square foot weight, but your windows will be measured per unit.
Dead loads are loads that will not change over time.
Live loads are loads such as human beings walking around, or in a garage structure, the cars driving around. Also, furniture, snow, plants, etc.. These are also calculated and they depend largely on the program of the various spaces.
A library or cafeteria is assumed to have a higher live load factor than a simple office (all those books). You can reference both your local building code as well as the international code council for rules pertaining to your project.
Live load + Dead load = Gravity Load which is basically the total weight of the building.