Rilles are long, narrow channels found on the surface of the Moon, primarily formed by ancient volcanic activity. They can vary in length and width, resembling dried riverbeds or cracks in the lunar landscape. These features are often associated with the Moon's maria, the dark basaltic plains created by past lava flows. There are two main types of rilles: sinuous rilles, which have winding, curvy shapes, and straight rilles, which are more linear. The study of rilles helps scientists understand the geological history of the Moon and the processes that shaped its surface over billions of years.