Torsion is the twisting of an object caused by a moment acting about the longitudinal axis of the object. A moment that tends to cause twisting is called torque.
The video below takes an in-depth look at torsion, including how to calculate the angle of twist, and the shear stresses and shear strains associated with torsional deformation. It also covers how ductile and brittle materials fail under torsional loads.