Facilitating Brainstorming

What or who is a facilitator? I believe one of the key skills to be a great consultant is to be an effective facilitator - in addition to consulting skills and subject matter expertise.

Came across this description on Facilitation at ram Charan's website from his latest book The Game Changer, where he talks about how to brainstorm.

Facilitators need to be skilled at group dynamics, able to read when the team is flagging or when it is hitting on all cylinders. They have to be patient, yet willing to exercise discipline if one person can't stop talking or is becoming aggressive. It is more a matter of personality than formal training, but it can't hurt to bring in people to watch a well-run brainstorming session to see how it works.