A leader is somebody whom people follow. A leader has authority. For people to accept a leader, his or her leadership must therefore be legitimate.
Because people follow a leader, the leader can make people do things. A leader can therefore make people work together towards a common goal.
An example of a leader is the head of government of a country. People within the government follow the leader, and citizens look to the head of government for inspiration and guidance. Sometimes, when a head of government lacks legitimacy, people will stop following him or her. The head of government is therefore no longer able to lead properly. This may result in a new election, or in worst case civil war.
A group with no leader is called leaderless.
References
Other websites
'The Vision Thing': How Effective Leaders Cultivate and Co-ordinate Social Identity in Order to Promote Followership Leadership Qualities