Second messengers are small molecules that transmit signals within cells after a receptor on the cell surface is activated. They play a crucial role in cellular communication, helping to relay information from hormones or other signaling molecules to target areas inside the cell. Common examples of second messengers include cyclic AMP (cAMP) and calcium ions (Ca²⁺). These molecules amplify the signal, allowing a single receptor activation to produce a significant cellular response. By activating various proteins and enzymes, second messengers help regulate processes such as metabolism, gene expression, and cell growth, making them essential for maintaining cellular function and homeostasis.