Ring Spun yarn is made by constantly rolling and thinning fibres, using a ‘ring’ for spinning. It uses longer fibres which means the end result is a more uneven yarn. It was used as method of production until the late 1970s, but because it is labour intensive and takes more time, ring spun denim was replaced by cheaper, open-end yarns. The rough and uneven look is now back in demand, because of its likeness to traditional vintage denim.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary