It is a transverse force, one part of the beam exerts on the other part at any cross section. It is equal to the algebraic sum of all the transverse forces (including the reaction) either to the left or to the right of the cross-section. In the case of horizontal beams, shear force is the unbalanced vertical force on the left or right side of the cross section.
It is the couple, one part of the beam exerts on the other part, at a cross-section. It is equal to the algebraic sum of moments of all forces (including that of reaction) either to the left or to the right of the cross-section about the section. In the other words, bending moment is the unbalancing moment of forces on any one side of the cross-section considered.
Below diagrams are explain the shear force and bending moment diagram for Cantilever Beam.
Concentrated load at the free end
Uniformly distributed load