More information can be found at energy.gov:
You have probably seen the home improvement shows with spray foam insulation being applied. You know it is sprayed on and expands almost instantly. But what is it?
Spray foam is a polyurethane foam that is essentially the same material used in every day products: furniture, ice chests, refrigerators, etc. Spray foam achieves its benefits in insulation compared to traditional fiberglass by addressing the three avenues of heat transfer: Conduction, Convection, and Radiation.
When you hear people talk about insulation, you here the term 'R-value' thrown around a lot. R-value is important for conductive heat transfer, the actual movement of heat from two materials touching each other, however it does not address the other two forms of heat transfer: convection and radiation. Convection is the movement of heat through a liquid or a gas in and around a material (think of a convection oven), and radiation is the transmission of heat via electromagnetic rays that causes surrounding areas to heat up (think of items in a car or attic heating up). A home with Spray Foam protects you from all three types of heat transfer, thus providing you with one, nice, comfortable home.