The barrel is a container that looks fairly average, but in truth it's one of the more unique items in the game.
They can neither be bought nor crafted. To obtain one you'll to collect an empty Glass Pitcher, then head to the docks in any major town or city. Use the pitcher on a barrel filled with water and the empty item can then be picked up. New water barrels appear each day.
Use a Pitcher of Water on a barrel to pour the contents into it. Doing this will cause the barrel to fall to the ground again, and make it impossible to move until it is once again emptied. These barrels used to stay on the same spot no matter how long it had been left there, unless someone came and removed the water from it. As of Publish 66, barrels containing water can be destroyed by the use of an axe.
Each barrel can hold the equivalent of 20 filled pitchers. If this is the amount poured in, then this is the amount that must be removed in order to move it again.
Many players have been known to "decorate" certain areas will barrels, or perhaps use them to spell out their names in remote areas of the game world. Placing a large amount of barrels in a well populated area typically results in their swift removal by those concerned about game lag.
While not commonly seen, many players falsely assume that barrels are rares, even to the extent that some unscrupulous players offer them for sale as such.