I like this site. It provides informative descriptions of biological processes with visuals to boot. This one addresses cell signaling. The following quote in blue is taken from the article:
The Notch signaling mechanism is an example of juxtacrine signalling in which two adjacent cells must make physical contact in order to communicate. This requirement for direct contact allows for very precise control of cell differentiation during embryonic development. In the worm Caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide. The choice of which cell continues to divide is controlled by competition of cell surface signals. One cell will happen to produce more of a cell surface protein that activates the Notch receptor on the adjacent cell. This activates a feedback system that reduces Notch expression in the cell that will differentiate and increases Notch on the surface of the cell that continues as a stem cell.