What is Leadership?

What is leadership? Most of the tomes I've read about leadership talk about the character of the leaders they are describing. Or they talk about the principles by which they attempt to live or lead. Regardles of which approach I still ask the question: Is it character, living principles or leadership principles? How do you know that someone is a good leader? Don't you know people who are good managers, but bad leaders? Or Good leaders, but bad managers? What makes or breaks a leader? How do you describe a good one sufficiently to teach or coach others to become good leaders?