When a deforming force is applied to a body, it changes the shape or size of the body by changing the normal positions of the molecules or atoms of the body.
As a result, an internal restoring force comes into play, which tends to bring the body back to its initial shape or size. This internal restoring force acting per unit area is called stress.
i.e. Stress = Restoring force / area
When a deforming force is applied on a body, there is a change in the shape or size of the body. The body is said to be strained or deformed. The ratio of change in dimension to the original dimension is called strain.
i.e. Strain being the ratio of two like quantities has no units and dimensions.