ICF Walls
Insulated concrete forms are hollow, block like structures that stack together like concrete block and are then filled with concrete. These forms are then left in place to function as insulation for the wall, resulting in a wall that is better insulating while also using less concrete than a standard poured concrete wall.1
Most ICFs are composed of virgin polystyrene, but there have been a variety of products on the market using ground up recycled polystyrene in a Portland cement matrix ("composite ICF"); unfortunately products seem to come and go as these composite ICFs are used more in residential building, and the largest market for ICFs overall is in commercial building. There is also at least two products (Faswall and Durisol) that use wood chips instead of polystyrene.
Rebar (as needed) is installed both vertically and horizontally thru the blocks as they are assembled. The blocks themselves are glued together, for example with spray foam. When the wall height is reached the concrete is typically poured with a boom pump truck. If you use one of the composite ICF types (ie wood chips or recycled styrofoam), you can stucco right over the blocks. Needless to say there is a learning curve, and the biggest risk is blowouts when the concrete cores are poured.
Since concrete has little insulating value, the insulating value of the finished wall is due to the ICF itself. A typical wall is at least 50% concrete, so the R-value of the wall is much less than other system which consist of a higher percentage of insulation for the equivalent thickness. The ICF industry has made various claims about high effective R-values, but as with any effective R-value due to mass, it doesn't work in most climates. ICF walls do have thermal mass, but it's not really in the right place (fully inside), but is thermally halfway between inside and outside. It will probably still function, but with less effect since its insulated from the inside, plus only half of the wall's insulation is on the outside, so whatever heat it hold will be lost faster than with conventional thermal mass. In order to get a true high R-value wall, you will need to add extra insulation, which is typically also foam.
Although it is possible to use ICFs for all a buildings walls, it generally only makes sense to use them below ground in the case where the space needs to be insulated since no other wall system works below ground. Exceptions to this would be areas that have an exceptionally high wind load, and areas where wood is not generally available, or someone who just wanted to avoid using wood. The typical use of ICFs would be in heated basements and daylight basements.
ICFs are often considers a "green" building material, but since both concrete and polystyrene come with a significant environmental burden, this is not an easy argument to make, other than as a direct replacement for solid concrete walls.
One of the main limitation of ICFs in the residential market has been cost--often quite a bit more than the standard solid foundation, but potentially competitive with an insulated one. Prices change often, so the only real way to find out is to price them.
Summary: There is a learning curve to installing these, with some systems being a bit harder than others, but in theory at least, insulated basements can be built for less with ICFs. For above grade walls, other wall systems generally perform better, and with less environmental concerns. For below grade or anywhere you need concrete anyhow and will insulate it,
Notes
1: The implication is that typical walls use more concrete than necessary, since the ICF add little strength, but I've never seen this topic discussed. It makes me think most residential concrete walls are overkill. The issue presumably is not the strength of the concrete, but the need to spread the load over a lot of soil, and avoiding the concrete cracking too much if its too thin.