The Leader attempts to gather others to a task or organization. This role is very social, and depends especially on the input and response of others. A Leader could be an active guild leader in Warcraft or EQ2, or assemble a team to take on dungeons or other adventures. He or she could lead a group of 4, or an organization of hundreds.
The Leader should always be seeking to actually lead, and should be directing people to have the most impact where ever they are. For example, a Leader in Face of Mankind could be a politician striving for control of a faction, or the leader of a small band of independent rogue operatives.
Any class can be a leader. The Leader gains fame by causing the most influence or having the most members in a guild or similar organization they control.