Basically, a single-ply thermoset roof consists of a roof membrane derived from oil and natural gas that's cured and bonded to roofing materials. These roofs are well-sealed and insulate the buildings they cover.
They're constructed in the factory as opposed to roofs that are constructed and sealed at the site. According to the Single Ply Roofing Association, they have a more consistent quality because they're not constructed on site.
According to the association, thermoset membranes are good at withstanding the damaging effects of sunlight. Usually the membranes are black, but manufacturers also make them in white, which is the best color for a cool roof.