Shear force is a force that acts perpendicular to the extension of a substance, such as the pressure of air along the wing of an airplane. There are two main types of shear forces: single shear and double shear. Single shear acts on a substance through one plane, while double shear acts through two planes. Shear forces are important in engineering and can cause failures, as seen in soil, beams, and columns if the shear strength is exceeded.