Let us first consider some rotational cases in daily life. Examples include screw driver action to fix a screw, opening or closing a medicine bottle cap etc.

Force acting on an object makes it to rotate in some cases. Here again torque is involved.

Let is have a rod of length r. When force F is applied as shown in the figure, the object rotates about an axis and this point is called as the pivot point, O.  The distance from the pivot point to the point where the force acts is called the moment arm, and is denoted by r. Now torque in this case is vector cross product of r and F which is Torque = vector r X vector F.

Thus, only external force acting on a body causes rotation, by the definition of torque as r X F and no internal force is involved here. Also unit of torque is N-m.