With each card played, you gain more information about the loot or sanctions that may be yours for the taking in different American cities. Basing upon this information, you send out the members of your gang to those cities: some are pros, some are wannabes.
Whoever has the most gang members in a city cashes the loot or undergoes the sanction. When going to Chicago, you have no choice but to split your takings with the boss himself: Al Capone. The further the game progresses, the higher the stakes are getting, especially in Chicago.
The Boss is essentially a card game (with a small game board for keeping scores) of deduction, bluffing, and a lot of backstabbing.