A rigid body is an object that does not deform under the influence of external forces. It maintains its shape and size, and all its particles move together when subjected to a force.
The center of mass is the point in a system where the total mass can be considered to be concentrated. It simplifies calculations involving motion and forces acting on an object.
Torque is a measure of how effectively a force causes rotation around an axis. It depends on both the magnitude and direction of the force relative to the axis.
The moment of inertia measures an object's resistance to changes in rotational motion. It depends on both the mass distribution and shape of the object.