Assassin missions in Assassin's Creed Brotherhood and Assassin's Creed Revelations have a difficulty rating. On some missions it's displayed as stars and on some as diamonds.
What's the difference? Which one is harder?
The diamond rated missions are just basic missions with pretty much standard experience and standard money gained.
Star rated missions are special in that they often come with additional perks. Perks such as added experience/money, additional income/commodities from an assassin controlled city and the like.
Additionally, star rated missions are more difficult for your assassins to handle compared to a diamond rated mission at the same level. You will usually need to provide more support from stronger/more assassins if you wish to be successful at that mission.