Insulation is a category of materials used in homes and other buildings to help them retain heat (or more technically, to help them resist the flow of heat).
The effectiveness of insulation is measured by its "R-Value." The higher a material's R-Value, the better it resists the flow of heat, and therefore, the better it insulates the building. For example, R-19 insulation retains heat better than R-11 insulation. Basically, the higher your home's insulation level, the cozier you'll feel. Insulation will prevent hot air from creeping inside of your home in the summer, and keep hot air from escaping your home during the winter.
There are now many green types of insulation available. Two popular types of green insulation are cellulose insulation, which is made from recycled paper, and denim insulation, which is made from recycled jeans. "Entourage" star Adrian Grenier used UltraTouch denim insulation in his Brooklyn brownstone.