A dune is a hill or ridge of sand formed by the wind, typically found in deserts or along coastlines. Dunes can vary in size and shape, depending on factors like wind direction and the availability of sand. They play a crucial role in protecting coastal areas from erosion and serve as habitats for various plants and animals. Dunes are often classified into different types, such as barchan, transverse, and longitudinal dunes, each with unique characteristics. The movement of sand within dunes can create dynamic landscapes, making them important for studying geology and ecology.