Folds are bends or curves in rock layers that occur due to tectonic forces within the Earth's crust. These forces can compress, stretch, or shear the rocks, leading to various types of folds, such as anticlines and synclines. Anticlines are upward-arching folds, while synclines are downward-arching. Folds can vary in size, from small-scale features visible in road cuts to large mountain ranges like the Himalayas. They play a significant role in geology, influencing the distribution of natural resources, such as oil and natural gas, and affecting landscape formation.