A barrel (タル,Taru?) is a container item which can hold various items that has appeared in the entire Super Smash Bros. series. This makes it similar to a crate. When one picks up a barrel, because they are so heavy, one will not be able to jump, attack, and one will move very slowly while holding it (with the exception of Donkey Kong). After one has picked up a barrel, one can throw it up, forward, or down. If a standing barrel is hit with a weak attack or lands on a sloped surface, it will roll across the stage.
In the original Super Smash Bros., the player cannot move while holding a barrel, unless they are playing as Donkey Kong. Interestingly, Donkey Kong can not throw Barrels below him. There is a 12.5% chance barrels, crates, and capsules will have explosives, so it is a good idea to throw them so they won't come back and possibly explode on the player.
In Melee, barrels can also very rarely release an Adventure Mode enemy, such as a ReDead, an Octorok, or a Goomba. When they are thrown at opponents, they can deal 18 to 22% damage and good knockback. When they explode once they have hit an opponent, they can deal around 45% damage and strong knockback. They contain only one item.
In Super Smash Bros. Brawl, barrels, along with Crates, have a different appearance depending on what stage the player is playing on. On a more futuristic stage, e.g. Lylat Cruise, they take on a futuristic appearance, and on more fantastical stages, e.g. Yoshi's Island, they look like wrapped-up presents.  Barrels also fly much farther and faster when struck. Additionally, if a rolling barrel comes to a rest, it stops being harmful to players and can be picked up a re-thrown, or attacked to send it flying/break it.
Like Crates, Barrels are often filled with items and occasionally explosive. Thrown Barrels may break on contact or go rolling along the ground to smash into characters. Like the Crate, the Barrel is heavy, so it will restrict the speed at which the character carrying it can move. This speed differs between characters.
Super Smash Bros., 04/99
Item holders similar to crates-- their appearance will also change to fit the area. When you throw a barrel, it won't shatter, but rather roll across the stage. When a rolling barrel hits a character, that character gets sent sailing. Barrels are heavy, so you'll need two hands to pick them up, and your foot speed will decrease too.
Super Smash Bros.
Super Smash Bros. Brawl
These barrels are crammed full of items, but they're also incredibly tough to break. No baby hits here—attacking too lightly will just send the barrel rolling away, making it a dangerous obstacle for both you and your foes. And if others break the barrel, they'll get the items you wanted!