We base the abstractions on the conceptual entities in a domain. An online bookstore, for example, will process many shipments to send orders to customers.
Definition: A class is an abstraction from a set of conceptual entities in a domain so that all the conceptual entities in the set have the same characteristics, and they all are subject to and behave according to the same rules and policies.
Put another way, a class abstracts common characteristics and common behavior.
Start out by asking yourself, ...