Roll clouds are a subset of arcus clouds, but defined by being completely visually detached from other cloud formations, including the parent storm that causes them (in the case of those like this that closely accompany thunderstorms). Roll clouds also sometimes arise from cold fronts, outflow boundaries, or sea breezes intersecting suitably moist air. They can be convective in character, like this one, when at least marginally unstable air is being lifted. Laminar (smooth) roll clouds—where stable but moist air is forced upwards along a wave crest—are more common with fronts and storm-distant outflows, but also can occur near convective storms.
2 NNE Hedgesville MT (19 Jun 13) Looking WSW
46.4904, -109.486