Shear force

A normal force is not some esoteric type of force---it is simply a force whose direction is perpendicular to a surface.

Shearing forces are unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction. When the forces are aligned into each other, they are called compression forces.

Two normal forces in opposite directions and not directly aligned along the same axis will cause shear forces.

If they are aligned, they are compression forces.

Unlike shear force, Shear stress acts in a parallel to the surface. ... It is termed as the ratio between force applied (F) to the cross sectional area (A) of the beam/structure. Shear stress acts in perpendicular direction to the normal stress applied on the material.
Definition of Shear Force And Shear Stress | Chegg.com
https://www.chegg.com/homework-help/definitions/shear-force-and-shear-stress-5

 

I don't seem to understand what that means in a practical application.

Stress is applied force per unit area.

If the force is normal to a surface, it is perpendicular to it.

To every force is an equal and opposite reaction when nothing moves as a result: such as a brick resting on a concrete floor.

Shear stress occurs when the two opposing forces are not in line with each other. A simple example would occur if you put that brick on the edge of a table, outside of the table legs. There, the legs provide the reaction to stop the brick crushing the table, but as they are not below the brick, the overhang is subject to a shearing force concentrated along the top edge of the leg. 


That simplifies a bit as it ignores the stress conditions within the overhanging area (the "cantilever") of table-top, trying to bend it; but essentially a shear is the result of two forces acting parallel to, but out of line with, each other