Leadership is the ability to obtain followers. Leadership is influence. You are a leader to the extent that people are following you.
Leadership is important because it influences the destiny of people.
Leadership also determines, to a large extent, what a group of people will be able to achieve. Leadership moves people towards common goals, principles and values. Good leadership enables people to work together well, and realise their potential. Poor leadership frustrates both people and projects. Poor leadership results in people being hurt.