Foamed concrete or
Autoclaved Aerated Concrete or
Autoclaved Cellular Concrete.
Aerated concrete is pretty much just what it sounds like it should be. It's concrete that has lots of little bubbles in it. This means that it is both lighter in weight and a much better insulator than is normal concrete.
Aerated concrete has been used since 1914, when it was developed in Sweden. It has long been used in Europe, and is slowly gaining in popularity and has recently taken off, including entering into America and other oversea markets. Structures built 75 years ago are still standing and in good shape, but the real reason for the current interest in aerated concrete is that it is good for the environment -- or at least as good as concrete can be.
It is lightweight, which saves on fuel during shipping, and well insulated (it has an R-value of about 10 for an 8-inch thickness, as compared to an r value of less than one for CMUs) and has a high thermal mass, which saves energy that would be spent on heating and cooling. In addition to this, it is durable and in some cases it's even cheaper than standard concrete.
Because aerated concrete needs to be cast under specific conditions, it is usually sold in pre-formed blocks. These blocks may be simple concrete blocks in the same form as cinder blocks, but it is sold in any number of forms, including large wall panels, interlocking blocks, pillars, and garden furniture. If you want to mold blocks on-site, some manufacturers have the resources to mix and mold aerated concrete in the field.
And on top of all that, it is also soft enough that it can be carved using woodworking tools -- but check with the manufacturer to make sure you're getting concrete of the right consistency.
Aerated concrete is made by mixing Portland cement with lime, silica sand (or recycled fly ash from coal-burning power plants), water, and aluminum powder in a mold. The aluminum and the concrete react together to form tiny hydrogen bubbles, which expand the concrete to about five times its original size. The resulting concrete is airtight, non-flammable, and of course, light.
It is sometimes referred to as Autoclaved Aerated Concrete, because an autoclave may be used to form the resulting concrete. "Cellular Concrete" refers to the small air-cells formed in the concrete.
Brands of aerated concrete include LITEBUILT (from the Pan Pacific Management Resources PTY LTD company), AERCON, Xella Aircrete (from Hebel) and e-crete (E-Crete LLC).