By ISO 5393 definition,
Hard joint is defined as low resistance to turning until its bolt or nut reaches its seating. After which, its resistance rapidly increases. It is usually referenced as an angle of less than 30° from the start of fastening to reaching the final torque. (torque rises from 10% to 100% and the rotation angle is less than 27°)
Eg of hard joints are metal or metal to gasket
Soft joint is defined as low resistance to turning until its bolt or nut reaches its seating. After which, its resistance slowly increases. It is usually reference as an angle of more than 720° from the start of fastening to reaching the final torque. (torque rises from 10% to 100% and the rotation is more than 650°)
Eg of soft joints are rubber or wood
In the following diagram, you will be able see the commonly faced situations – Over tightening hard joint and under tightening soft joint thus not reaching its optimal performance.