A grain silo can be defined as a structure created to store materials in bulk. They are usually tall and cylindrical, but can also be constructed in the form of warehouses, domes, or large elongated bags. They primarily hold grain, seed, or silage. Silage is fodder that is harvested green, and then stored and left to ferment in a grain silo. This fodder is then used to feed livestock, or as a biofuel, and is usually grass, alfalfa, sorghum, oats, or maize.
The three primary types of grainsilos are bag silos, bunker silos, and tower silos. Bag silos are an inexpensive way to store grain and silage. They are made out of a heavy, durable plastic, usually 6-8 feet (2-2 ½ meters) in diameter, and can be of any length. They are often used for overflows, when more storage space for grain is necessary.