Stress
Stress is the force per unit area acting on a plane within a body. Six values are required to characterize completely the stress at a point: three normal components and three shear components.

Normal stress
The normal stress is that stress component perpendicular to a given plane. If you lean against a door after you close it, you are applying normal stress to the door. Normal stress can either be compressional or tensional.

Compressional Stress
Compressional stress is the stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

Tensional Stress
Tensional stress is the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

Shear Stress
Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.