set designers for plays

I always call them "flats" but there are other terms.

http://en.wikipedia.org/wiki/Flats_(theatre)

we always called them flats too..unless you painted on canvas and sretched it taut..then we called em backdrops

Yep, those are "flats" and they are sort of tricky to put together if you're new to stage design and production. You put a lot of work getting them set up only to have to strike the set a month or two later (depending on how long the show lasts they may stay up longer).