Stress is the force per unit area acting on millions of atoms in a given plane of a material. Thus, stress can be expressed as the force acting over an imaginary surface area inside the material. The unit for stress is Newton/m2 or Pascal. Since 1 Pa is a very low stress, we often use the unit of 1 N/ mm2, which is 1000000 Pa or simply 1 MPa (MegaPascal). One MPa is slightly less than 1 lb/mm2.
Strain is related to the displacement of the atoms in relationship to their original neutral position. Thus, strain can be expressed as change in length divided by the original length. Strain does not have a unit.
If a force acting on a body tries to separate the atoms inside the body, the resulting stress is a so called tensile stress, while if the force tries to move the atoms closer together the resulting stress is a so called compressive stress. Tensile and compressive stresses are so called principal stresses. A combination of tensile and compressive stresses result in a so called shear stress. Shear stress implies that a layer of atoms is displaced in a sideward direction and that the force acts in a direction that is parallel to the slip plane.
