Class diagrams show the static structure of the systems. Classes define the properties of the objects which belong to them. These include:

	Attributes - (second container) the data properties of the classes including type, default value and constraints.
	Operations - (third container) the signature of the functionality that can be applied to the objects of the classes including parameters, parameter types, parameter constraints, return types and the semantics.
	Associations - (solid lines between classes) the references, contained within the objects of the classes, to other objects, enabling interaction with those objects.