Since the release of Old Gods DLC, and 1.10 patch, looting is a mechanic.
Looting can only be carried out by members that have any of the following:
A culture that allows raiding, for example, Norse or Norman.
A government that allows raiding, primarily tribalism.
A religion that allows raiding, this is mostly the pagan religions. In the early starts, Germanic pagans are the most common raiders, as rulers of that religion are the only rulers with shipbuilders.
Raiding will make your army hostile to any troops of the country you are raiding. This will mean that they can, and often will attack you while you are raiding.
Each province has an amount of gold that potentially can be looted by raiders. The amount varies depending on the wealth of the province. This can be seen by the yellow bar on the holding tab. The amount of gold a conquering army receives after the siege is displayed on the siege results screen.
Fortification and defense buildings in the holding decrease amount of gold given to raiders that has not yet successfully sieged the holding. If the holding is successfully sieged or assaulted all the gold from that holding will be taken. If a holding is sacked by siege or assault the province will be immune to raids for a short period of time.
When declaring war against a country, raiders will continue to raid and all sieges performed by them will not contribute to warscore.
When looting is carried out from a boat and the raiders are offloaded to a province that you do not border any held territories you will not immediately gain the gold. The raiders will keep the gold and when they re-board their raiding ship they will "store" gold on the ship. Ships can hold 10 gold per ship. To add this raided gold to your treasury you must dock the ship at one of your provinces. Once docking you will receive all the gold you raided and an equal amount of prestige.