A highly viscous aqueous solution, used as a hair styling aid to avoid frizz, to hold curls and waves, and even to keep short spikes standing upright.
The hair gel sitting in my bathroom cabinet of doooooooom at the moment appears to have PVP copolymer as a main ingredient. This chemical is a copolymer of vinylpyrrolidone with vinylacetate, and can serve as a film coating agent in the pharmaceutical industry, a viscosity-enhancing agent in printing ink, a binder in paints, and a stabilizer for the dispersion of dyes.
Hair gel also usually contains additional ingredients to add a pleasant fragrance, retard separation of the solution, and to prevent spoilage by bacterial growth.
Hair gel, when combined with South Florida's extremely high humidity, summer heat, and sweat, is also a GREAT substance for the generation of icky slime.
Source for the nifty information on PVP: http://www.peakchem.com/products/PVPA.htm