They are large panels (up to 4.3m x 1.5m or there abouts) for construction.
In general they are a very strong insulating material (expanded polystyrene) sandwiched between boards of OSB, like the boarding they use on roofs. They are the inner boarding (ready for plasterboard) an outer board (ready for rendering) and insulation all in one.
The panels are fabricated to order, then craned into position on the site. They negate the need for a separate frame to build around.
From a greeny point of view...
We have found a company that manufacture the panels about 60 miles away.
Because the boards include high performing insulation it is easier to add further insulation on the inside or outside, if you want to (some times you have to rely on the u-values of outr coatings and inner building materials to boost your insulation)
The wood is made from forest waste material, from forest management practices... and apparently is soil association approved
The insulation material is up to 98% air, and also manufactured on site 60 miles away.
Build time is greatly reduced, (saving energy, transport costs and cost associated with living away from home) panels can be fitted in place in a matter of days, with local trades (or indeed us) completing the inside and outer finishes.
We don't yet know if our extension can be built using the technique, but we are possitive. We think it would be ideal for us as our kitchen and bathroom will be demolished at the beginning of the building work and we don't have anywhere else to live (sounds fun huh?) So a quick turn around would be ideal.
Where we are, we get weeks of rain on end, which would halt any normal building work. From what I understand these panels can be installed in less favorable conditions (ie, maybe drizzle or lower temps)
Heres the wiki page if you want to read more
http://en.wikipedia.org/wiki/Structural_insulated_panel