Secondary messenger systems are crucial intracellular signaling mechanisms that amplify and propagate signals from the cell surface to the interior, enabling cells to respond to external stimuli with appropriate physiological changes. They operate through a variety of molecules, such as cyclic AMP (cAMP), cyclic GMP (cGMP), inositol trisphosphate (IP3), diacylglycerol (DAG), and calcium ions, which are activated in response to primary messengers like hormones and neurotransmitters.