The restoring force is often referred to in simple harmonic motion.
An idealized spring exerts a force proportional to the amount of deformation of the spring from its equilibrium length, exerted in a direction oppose the deformation.
The amount of force can be determined by multiplying the spring constant, characteristic of the spring, by the amount of stretch, also known as Hooke's Law.
When a pendulum is not swinging all the forces acting on it are in equilibrium.
The force due to gravity and the mass of the object at the end of the pendulum is equal to the tension in the string holding the object up.