Describe basic object-oriented concepts in the context of structural modeling

What will be an ideal response?

Classes and objects are black boxes: their outside is visible while their inside is not. This characteristic is achieved through encapsulation, the enclosing of data and processes within one single unit. Information hiding, which follows encapsulation, ensures that the inner entities and the workings of the object are concealed (and safe) from outside entities.
Encapsulation results in two spaces: public (the outside) and private (the inside). What the object offers to the public is called its interface. The interface is both the services that an object provides and the form that these services take. Since, in an information system, the only reason for an object's existence is the services that it offers to other entities, the interface of the object is characterized as a "contract" or a binding agreement.
Under this "contract," an object assumes certain responsibilities. The responsibilities fall into two categories: what the object "knows", or attributes, and what the object "does", or operations. A class defines the responsibilities and its instances (the objects) carry them out.
Even though an object is a building block of an information system, it might be composed of other objects. Such objects are called composite objects. A collection object is a composite object that is made up of objects instantiated from the same class. Composite objects may belong to a hierarchy: a component of a composite object may itself be a composite object.