A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady state, although the liquid phase may still diffuse through this system.
An upturned vial of hair gel
Silica gel
Hydrogel of a superabsorbent polymer
Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.
A block of silica aerogel in a hand.
A flower resting on a piece of silica aerogel, which is suspended over a flame from a Bunsen burner. Aerogels have excellent insulating properties, and the flower is protected from the heat of the flame.
A 2.5 kg brick is supported by a piece of aerogel with a mass of 2 g.
The "Stardust" dust collector with aerogel blocks. (NASA)