Kevlar, Cuben Fibre and other high tech materials are the buzz words. There are lower tech options such as Dacron, cotton and event old school authentic tanbark. There is a further alternative- poly tarp. Yes, poly tarp similar to the ubiquitous blue tarp used to cover all sorts of things. Polysails are made of white poly tarp rather than familiar blue.
White tarp has about twice the weave density as compared to blue.
Poly Tarp sailmaker Dave Gray sells his poly tarp sail kits the world over. His main market is builders of small, homebuilt wooden boats. For boats under 24 feet and 1000 pounds these Poly Tarp sails can make an inexpensive alternative to higher tech sails.