In Simula,
classes are blueprints for defining a set of similar objects. Thus, objects that
have the same attributes and respond to the same messages can be grouped together to
form a class. The attributes and associated methods are defined once for the class rather
than separately for each object. For example, all branch objects would be described by a
single Branch class. The objects in a class are called instances of the class. Each instance
has its own value(s) for each attribute, but shares the same attribute names and methods
with other instances of the class, as illustrated in Figure 25.4.