Great Leaders (two of them only Agammemnon and David) appear randomly from culture. They are best used around barracks. Settle them in the same city as a barracks and you get a free upgrade with each new unit which can make you crank out ridiculously powerful units.
Great Generals are what he's referring to and Hertsh is right, they appear randomnly after a great battle where the odds are against you or even. They are unnamed and add a 50% bonus to your unit and all units in that space.