Cells in a multicellular organism constantly influence each other's behaviour — triggering growth, halting division, changing what genes get expressed — without needing to be physically connected or touching. Cell signalling is the general term for how this happens: a sending cell releases a chemical messenger, that messenger travels to a target cell, and a receptor on or in the target cell recognises the messenger specifically and triggers a response, all without the two cells needing direct physical contact for the message to genuinely change the receiving cell's behaviour.
Different signalling ranges for different jobs
Cell signalling operates across several distinct ranges depending on what job it's doing. Endocrine signalling releases hormones into the bloodstream, letting a signal travel throughout the entire body to reach distant target cells, useful for coordinating slower, body-wide processes like growth or metabolism. Paracrine signalling releases a messenger that acts only on nearby cells, useful for coordinating activity within a localised tissue without the signal reaching unrelated distant cells that don't need it. Some signalling happens through direct contact between adjacent cells' surface proteins, without any messenger being released into the surrounding space at all. Whichever range is involved, the same basic principle holds: a chemical or molecular signal, not physical force or a mechanical connection, is what actually carries the message from one cell to another.
A receptor's specificity is what makes the message meaningful
A chemical messenger drifting past a cell only produces an effect if that cell has a receptor shaped to specifically recognise and bind that particular messenger — a cell without the matching receptor is effectively deaf to a signal that a neighbouring cell with the right receptor would respond to strongly, even though both cells are bathed in the exact same extracellular environment. When a messenger does bind its matching receptor, that binding event triggers a cascade of changes inside the target cell — often a chain of intermediate signalling molecules relaying and amplifying the original signal — that ultimately changes what the cell does: which genes it expresses, whether it divides, or how its metabolism shifts. The specificity of the receptor, not the messenger's ability to physically reach a cell, is what determines which cells actually receive and act on a given signal.
What we're still unsure about
The general categories of cell signalling — endocrine, paracrine, and contact-dependent — and the principle that receptor specificity determines which cells respond to a given signal, are well established cell biology. What remains a substantial and active area of research is mapping the full detail of the intracellular signalling cascades a given receptor triggers, since a single receptor can often set off multiple different downstream pathways depending on the cell type and context, and researchers are still working out the complete picture for many of these pathways, particularly where a receptor's downstream effects differ meaningfully between healthy tissue and disease states like cancer.
This sits inside Cell Signalling & Receptor Pathways, one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.