Stiffness is a measure of how much a material can resist deformation based on a give load, it's dependent on the geometric properties of the material; it's measured in the linear region of the stress strain curve. Strength is a measure of how much a material is able to withstand stresses, like a shear stress though a section or a compressive and tensile stress depending on the load. It can be measured at different points in the stress stress curves and there a variety of important strenghts; yield, ultimate, failure stress.