From Aerogel.org

Aerogels are an amazing group of solids. First invented in 1929 by Dr. Samuel Kistler, aerogels have a wide variety of interesting traits and functions.
On aerogel.org, some of the traits of aerogels are explained:

Aerogels are the world's lightest solid materials, composed of up to 99.98% air by volume. Aerogels are a diverse class of amazing materials with properties unlike anything else. Transparent superinsulating silica aerogels exhibit the lowest thermal conductivity of any solid known. Ultrahigh surface area carbon aerogels power today's fast-charging supercapacitors. And ultrastrong, bendable x-aerogels are the lowest-density structural materials ever developed.

To learn more about aerogels, click here.