CRC card

Class Responsibility Collaborator (CRC cards) are a brainstorming tool used in the design of object-oriented software. They were proposed by Ward Cunningham and Kent Beck. They are typically used when first determining which classes are needed and how they will interact.
CRC cards are usually created from index cards on which are written:

The class name
Its Super and Sub classes (if applicable)
The responsibilities of the class.
The names of other classes with which the class will collaborate to fulfill its responsibilities.
Author

Using a small card keeps the complexity of the design at a minimum. It focuses the designer on the essentials of the class and prevents him from getting into its details and inner workings at a time when such detail is probably counter-productive. It also forces the designer to refrain from giving the class too many responsibilities. Because the cards are portable, they can easily be laid out on a table and re-arranged while discussing a design with other people.

A common method to determine what cards should be created is to read a specification for the program being designed and consider if each noun should be a class and if each verb should be a responsibility of the noun or class to which it belongs. Naturally, the existence of a noun or verb does not require a class or responsibility in the program, but it is considered a good starting point.