Stress refers to the forces exerted by particles of a material exert on one another. 

For example, consider, a loaded truck standing on a bridge.  The weight of the truck exerts a force on the material in the bridge.  The bridge is thus stressed by the weight of the truck.

Stress can be due to an external force or due to internal flaws and defects in the material. Examples of materials having internal stresses are prestressed concrete, tempered glass, etc. 

Stress is defined as the force per unit cross sectional area. 

Stress = Force/ Area

Stress can be expressed in many units such as kg/sq.m, pascals, pounds/sq. inch, etc. 

Normal Stress and Shear Stress
The stress on an object can be resolved into two types stresses  They are normal stress and shear stress.

Normal Stress is the stress  that is normal to the plane of the material. Shear Stress is the stress that is parallel to the plane of the material.

The deformation suffered by the body as a result of stress is called strain.