Shear force
is an internal force in any material which is usually caused by any external force acting perpendicular to the material, or a force which has a component acting tangent to the material.
For example
Take a ruler or a block of wood, and put it in table surface. Pushing the ruler or the block of wood in the downwards direction, will create a shear force inside the block of wood or the ruler.
Picture
Reference
(Answers.com)
