A Committee is a group of volunteers that are allowed to email the rest of the group all at once. It does need to be a committee in the formal sense of the word.
Volunteers cannot sign up to be on a Committee; they must be placed on it by an administrator. Volunteers can only see a Committee if they have been added to it and if you have enabled the members to email each other.
A volunteer can send an email to the members of the Committee from their CONTACT tab in MyVolunteerPage.com without seeing the names or email addresses of the members of that Committee. When the recipient reads the message (in their own email provider), they will see the name and email address of the sender, but no information will be displayed to them about any other potential recipients of the message.
As an administrator, you can also search for people by their Committee membership and use it as a filter when assigning volunteers to an Activity.