Definition: an Affinity Diagram is a technique used to organize and group ideas or information.
An AD is frequently used following a brainstorming session to sort and make sense of all the ideas that have been previously expressed.
The AD process:
1. Write each idea on a card or post-it note
2. Look for ideas that seem to be related, based on natural relationships, association.
3. Stick (glue) all cards into groups on a wall
4. Name each group
5. Optionally split large groups in sub groups
Alternatively an Ishikawa Diagram or the Six Thinking Hats can be applied to group ideas.