Shear force is a force that acts on an object in a direction perpendicular to the extension of the object. For example, wind pushing against a tree is a shear force. Shear force results in shear stress, which can eventually snap or break an object.
Shear forces often have to be taken into account during the design and build stages of many engineering projects. While many forces may act on any given structure or object, shear forces tend to result in catastrophic failure before a structure shows signs of bending or distorting. Shear forces cause shear stress in an object. Shear stress is the result of two forces acting on an object in opposite directions. For instance, a bridge may fail as a result of shear forces. Gravity and weight from traffic apply force downward on the bridge while support struts and cables apply force upwards. These two opposing forces combine to create shear stress and can result in a bridge snapping at the point where the opposing forces meet.
Wind against an airplane wing is another example of shear force. The wind pushes back on the wing while the plane and engine apply forward motion to the wing. If the wing is not designed with shear forces in mind, it fails and is torn from the plane.