Cell signaling allows cells to communicate and respond to their environment. Extracellular signaling molecules bind to receptors on cells, which then activate intracellular responses. Prokaryotes use quorum sensing to coordinate behaviors based on population density. Eukaryotic signaling is more complex, with over 1500 receptor types in humans. Signaling pathways are classified by the signaling molecule (hormone, neurotransmitter, cytokine) and can cause changes like gene expression, enzyme activity, or cell movement. Defects in signaling can lead to diseases, making cell signaling an important area of research.