A tarpaulin (sometimes shortened to tarp) is a large sheet of heavy fabric, such as canvas. Modern tarps are usually made of woven polyethylene and have reinforced grommets at the corners and sides. As they are quite waterproof the are frequently used as tent flies.
Tarps are a common sight at SCA camping events, owing to their general usefulness, but since most polyethylene varieties are brightly coloured they look (and sound) quite artificial; this makes them unpopular with more stringent re-enactment groups.
Because of the prevalence of polyethylene tarps, tarpaulins made of more traditional materials are frequently referred to as specifically "canvas" tarps.