An object contains encapsulated data and procedures grouped together to represent an entity. The ‘object interface’, how the object can be interacted with, is also defined. An object-oriented program is described by the interaction of these objects. Object-oriented design is the discipline of defining the objects and their interactions to solve a problem that was identified and documented during object-oriented analysis.

Object-oriented design is a method of design encompassing the process of object-oriented decomposition and a notation for depicting both logical and physical as well as state and dynamic models of the system under design.

http://en.wikipedia.org/wiki/Object-oriented_design
http://en.wikipedia.org/wiki/GRASP_(object-oriented_design)