Fishbone Diagram (Ishikawa Diagram or Cause and Effect Diagram) - is a graphical tool to depict the potential causes for a specific problem/event/issue. Potential causes are identified as an outcome of the brainstorming activity.
The problem is shown as the head of the fish (on the right) and causes become the bones (on the left). Ribs branch off the main back bone as categories in which the potential causes are clubbed. The ribs can be sub-branched for root causes to as many levels as required.
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Mohan PB on 17th November 2017.