There are two kinds of great leaders: Military and Scientific. A Military leader is born in combat. This can occur when an elite unit wins a battle. A Military Leader can be used to either create an army or to hurry production of an Improvement or Small Wonder in a city. When a military leader does either of these actions he disappears. A Scientific leader comes about as a result of research. Each time a technology is researched for the first time by any civilization there is a chance that a Scientific leader will arise (this chance is increased slightly if the researching civilization has the "scientific" strength. A Scientific Leader can be used to increase the scientific output of all cities of his civilization, thus starting an Age of Science. Simply move the scientific leader to your capital and press the "Increase Scientific Output" button. A Scientific leader can also be used to hurry the construction of any improvement (including a Great Wonder) of the city he's currently in. When the leader does either of these actions he disappears.