When combat is over, the attacker loots the city, taking resources and any food in unharvested fields. They also have a chance to capture a Noble from within the Keep. If they capture a Noble they take them back to their Keep's dungeon and can then hold them, release them, ransom them back to you, or schedule them for execution. 

If you reinforce an attacker and they are victorious, the loot is shared between everyone on that side based on how much damage their troops did as a percentage of the total damage done.