Stress and strain are both physical properties of a material or body part that when it is put under pressure or weight is applied. It can become deformed, crushed, or broken. A strain occurs when the material or body part is impaired from overuse or overexertion.
For the body, it would be an injury from lifting something too heavy and putting abnormal tension, which can tear tissue and ligaments. Whereas in the case of a building, there could be cracking or tilting due to the stress of uneven weight or not enough structural integrity to hold the weight.
Used in physics, stress is defined as a force that becomes the cause of change in a physical body. This change produces tension, which causes the body to deform. Also used in physics, strain is the change that occurs to an object when stress occurs.
The object can change in either form or shape. The forces that cause the body to deform are strains. Stress can happen without any strain, but strain can only exist with stress. Another difference between them comes in measuring. Strain is unable to be measured, but stress can. It even has its own unit of measurement.