Loot is the amount of resources you gathered (stole) during a fight. The amount of loot you gain is not dependent on what building(s) you attack.
Looting normally takes place during rounds 6 to 20. Each round units will normally take up to 1/15th their carry capacity. With the Greed Is Good technology this is improved by one round per rank of greed is good. With the full 10 ranks in greed is good, troops will loot 1/5th their carry capacity per round over rounds 6 to 10. This is valuable when your raids are losing stamina due to destroying enemy buildings. When attacking Barbarian camps if the camp is destroyed your troops wil be awarded any remaining loot at the barbarian camp as bonus loot regardless of how many rounds the battle lasted.
Bonus Loot is gained from destroying buildings and losing troops. There is a limit on the amount of bonus loot you can receive during a battle which can be boosted using the Sun Dance technology.
Units can only carry a certain amount of resources, which is listed under Carry. This is their total carry and is resource independent (for example a level 1 Hoplite has a carry of 20, this means that it can carry up to 20 gold, wood, crop, and iron each).
For the carry stat of each unit, check their page on the database.