Complex projects can be planned with colleagues who are thousands of miles, and several time zones away. Private mail and public announcements can be exchanged. Mail is very similar to the mail you send through the post office. Only the people you send mail to can read it. The recipients can in turn, reply to you through the mail.
Conferences are similar to meeting rooms or classrooms.
If someone speaks in a conference, all the people in that particular conference can hear . Conferences are generally used to work on projects or discuss certain topics with a group. Mail is normally used for private discussion between a few members.