Who uses it: Historians
What it means: The idea that -- in the words of Thomas Carlyle -- "history is the essence of innumerable biographies," that the will and actions of specific, individual human beings are the primary cause of world events.
How you can use it: To explain our current President.
The opposite of this idea is Hegel's theory of history as the actualization of a universal mind -- a separate force that finds its way despite the will of individuals.
You could dismiss the whole idea of "theories of history" as academic self-indulgence, except that the belief in a particular set of causes and effects shapes your decision-making process.
George W. Bush believes in the Great Man theory and in its corollary, which says that leaders are born, not made. Therefore, the war on terror requires only that we eliminate certain prime movers: Saddam Hussein, now Abu Musab al-Zarqawi, and one day Osama bin Laden.
So now al-Zarqawi is dead. Does this mean the U.S. troops get to come home now?