Containers are items in Forge. They come in a variety of types.
Boxes are the most common Forge item. Boxes come in a variety of sizes. You can get small or double sized boxes, both come with or without holes. These can be found on many maps. You can stack these on top of each other than delete the bottom one and the top will float. There are a few other methods to do this.
Containers are also a common Forge item. These look like boxes, but usually come in different colors such as red, green, and blue. These usually also come in the same sizes that boxes do. The big differences between containers and boxes is that containers can't float. Containers can also be moved in a non-Forge game. This can be done by explosions or by being hit with a vehicle.