A spoiler helps aid airflow on the car, usually to increase downforce (with th eside effect of increasing drag). They are often fitted to coupes. Spoilers were part of the Datsun 1200 FIA homologation. Sometimes they are called "wings" but a true wing is something different. The purpose of a spoiler is to aid airflow, not to create lift but usually to counter it.
The "FIA" style rear spoiler -- aka Bobtail, Ducktail, etc.
Classic old-school "bobtail spoiler"
Also see: Air dam (from air dam "spoilers")
Suppliers
See Body panels
Materials
Most spoilers are made out of wikipedia:fiberglas.
Also see: Parts Reference