The upstream signaling pathway is triggered by the binding of a signaling molecule, a ligand, to a receiving molecule, a receptor. Receptors and ligands exist in many different forms, and only recognize/bond to particular molecules. Upstream extracellular signaling transduce a  variety of intracellular cascades.
Receptors and ligands are common upstream signaling molecules that dictate the downstream elements of the signal pathway. A plethora of different factors affect which ligands bind to which receptors and the downstream cellular response that they initiate.