A number of things regarded as forming one group through the possession of similar qualities; a kind; sort. (Delbridge & Bernard 1998) Classes are the focus of most ontologies. They describe concepts in the domain. For example, the class of public health 'functions' represents all public health functions. Specific functions, for example, 'protect from threats to health', are instances of this class. A class can have subclasses that represent concepts that are more specific than the superclass. For example, we can divide the class of all public health 'functions' into 'assess...', 'protect...' and 'promote...' functions. 67 Alternatively, we can divide the class of all public health functions into primary and secondary functions. (Noy & McGuinness 2001 : 3)