I think @EE_Elephterion 's answer was pretty short and explanatory.
There are three types of worlds:
1.- The normal worlds, also called "new economy" (there is no attack by the forgotten ones and there are no losses in the effectiveness of the attack on other targets)
2.- The uprising worlds, where the forgotten launch attacks against our bases regardless of our faction or alliance. The number of ongoing Forgotten Attacks depends on the number of Forgotten Bases around you. The greater the number of forgotten bases, the greater the number of waves (consecutive attacks).
3.- The worlds with morale, where a percentage of the attack level loss of your troops is defined based on the size and strength of the defenses of the target you are attacking when compared to yours (reduces the efficiency of your attacks). Defense bigger than your attack, your troops get scared and lose some of their morale and are less effective. The greater the difference against, the greater loss of morale and effectiveness of the attacks.
