The force exerted by a spring when it is compressed or stretched is described by Hooke's Law. This law states that the force is directly proportional to the displacement of the spring from its equilibrium position, expressed as \( F = -kx \), where \( F \) is the force, \( k \) is the spring constant, and \( x \) is the displacement. This principle is a fundamental aspect of mechanics and is used in various applications involving elastic materials.