Torque is what causes rotational speed to change. Just as greater net forces cause greater linear accelerations, greater torques cause greater rotational or angular accelerations.
Torque is composed two different things: a force and a lever arm. The only component of force that produces torque is the component that is perpendicular to the axis of rotation. The line of action is an extended line drawn colinear with the force. The lever arm is the distance between the line of action and the axis of rotation, measured on a line that is perpendicular to both.
