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 splitting 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 (but comes with small game board for keeping scores) of deduction, bluffing and a lot of backstabbing.