They are different measurements of a material under a load.

Stress is the amount of Force per area. If you pull on a bar that has an area of 1/2 square inches with a 5000 pound force then the Stress would be the Force divided by the Area or

Stress = 5000 LB /divided by/ 1/2 In2 = 10,000 Pounds per Sq Inch (psi)

Strain is the measurement of the deflection of a material under a load. This measurement is very small and it is described in terms of Inches of deflection per the original length in Inches of the part.

Strain is measured as In/In. Under a given load, the stress will be present and the part will deflect. The chair you are sitting in has deflected ever so slightly while carrying your weight.