Class vs. Type vs. Category

Definition

a category is an intensional set (specifying necessary and sufficient conditions) with criteria for defining membership. A class is an extensional set where membership is explicitly asserted and specific properties can be defined as necessary.

in RDF, "type" is the class of an instance; Sub-classes can be said to be types of Classes but better to use subClassOf, e.g.:
- Car hasSubclass Ford #preferred
- Car type Ford #discouraged
- My old mustange type Ford #preferred
RDF Core Schema says "type" states that a resource is an instance of a class
type is problematic in informal speech; can mean instance of a class OR subClass of a Class
- e.g. Ford is a type of car, but Mustang is a type of Ford and convertible is a type of Mustang
  - Ford is a type of car manufacturer though
type feels a little less formal, but outside of programming they mean pretty much the same thing

A class is a category or classification used to describe a set of objects. So a class is a category, but it is also a set of objects.

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