Strength: When external forces are applied on a metal, breaking (brittle metal) or yielding (ductile metal) may occur. Strength is the property of a metal by which it resists external force without breaking or yielding.
Stiffness: When an external force is applied on a metal, it develops an internal resistance. The internal resistance developed per unit area is called stress. Stiffness is the ability of a metal to resist deformation under stress.
Toughness: When a huge external force is applied on a metal, the metal will experience fracture. Toughness is the ability of a metal to resist fracture.
Brittleness is a property of material which breaks with little or no deformation.