Spray polyurethane foam (SPF) is a spray-applied plastic that can form a continuous insulation and air sealing barrier on walls, roofs, around corners, and on all contoured surfaces. It is made by mixing and reacting unique liquid components at the job site to create foam. The liquids react very quickly when mixed, expanding on contact to create foam that insulates, seals gaps, and can form moisture and vapor barriers. SPF insulation is known to resist heat transfer extremely well, and it offers a highly effective solution in reducing unwanted air infiltration through cracks, seams, and joints. (From http://www.whysprayfoam.org.)
You can learn more by clicking on the picture below to watch the History Channel preview spray polyurethane foam on Modern Marvels.