Ward Cunningham calls his invention CRC or class, responsibilities and collaborators "a way of describing objects". After all, much of OOAD is to think about software inhabited by various roles (or classes), who interact with each other as collaborators, fulfilling their respective responsibilities. The beauty of CRC is its simplicity: Write on small index cards and shuffle them around as needed. Another beauty of CRC is its wide applicability. CRC's usefulness for designing implementation is obvious. Mostly overlooked aspect of CRC is its usefulness for high level activities like requirements analysis, use cases design and even setting up engineering process.

References: The original CRC cards "Object Design: Roles, Responsibilities, and Collaborations" by Rebecca Wirfs-Brock and Alan McKean