A class is a set of objects that share a common structure and a common behavior - Grady Booch

is a style of enterprise application development and integration, based on using automated tools to build system independent models and transform them into efficient implementations.

Every software system needs to have a simple yet powerful organizational philosophy (think of it as the software equivalent of a sound bite that describes the system's architecture)... [A] step in [the] development process is to articulate this architectural framework, so that we might have a stable foundation upon which to evolve the system's function points.

The object-oriented paradigm is useful when building software systems where there is a hierarchy defined as a ranking or ordering of abstractions.