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.
Classic old-school "bobtail spoiler"
Also see: Air dam (from air dam "spoilers")
Materials
Most spoilers are made out of wikipedia:fiberglas. Modern ones are made out of wikipedia:ABS plastic.
Also see: Parts Reference