What is the difference between hard, tough, strong and stiff materials? Can someone please give me definitions of these terms and examples of materials which show these properties?

Hint: The above characteristics i.e. harness, toughness, strength and the stiffness are the properties of matter, that determine their behavior on action of force. All the above properties enable selecting the appropriate materials in case of engineering design. Hence the above properties can be defined behavior of the material on the action of force and the respective change in properties.

Complete step-by-step solution:
Properties like hardness, toughness, strength and the stiffness are mechanical properties of matter. These properties of materials define its behavior under applied forces.
Strength is defined as the ability or the capacity of a material to withstand or support a load without a fracture. The strength of a material gives us the idea up to what extent the specific material can bear the load without breaking.
Hardness of a material is the ability to resist scratches, penetration and also plastic deformation. An example of such material is diamond which cannot easily be scratched or cannot be broken into pieces i.e. rather penetrates under the action of hydraulic press.
Toughness is the ability of a material to withstand shock loading or impact fracture occurs. This property gives us an idea whether if we throw something on a material what is its tendency to break under the given impact.
Stiffness of a material is the force required to change the dimension of the material by one unit. Stiffness gives us an idea what is the possibility of the change in length of a material under the action of force.

Note: It is to be noted that stiffness is a geometrical property. To understand this better, take a piece of paper and start folding it. Initially it is very easy to fold the paper as it is less stiff. But as we keep on increasing the number of folds the stiffness increases as a result it becomes difficult to fold the paper.