Signal transduction or cell signaling involves the conversion of information (transduction) contained in a receptor's activation by a ligand (e.g. a hormone, a neurotransmitter, a drug) into an intracellular signal that cells interpret in order to produce an appropriate response. Signal transduction pathways like the one above consist of a Receptor which binds a ligand (1st messenger).  The receptor-ligand interaction leads to a conformational change in the receptor,  information that is processed inside cells to produce a second messenger which is often results from the activation of an enzyme (e.g. adenylyl cyclase which increases cyclic adenosine monophosphate (cAMP) levels inside the cell).  These second messengers lead to activation of additional "downstream" effector molecules like protein kinases which covalently modify proteins through phosphorylation (e.g. the glucose transporter), changing that protein's activity, the cellular response that occurs in response to the ligand.

Signal transduction pathways are initially assessed by examining how a signal is transferred linearly from one component to another.  This is particularly useful in identifying the major players involved in a receptor's signaling pathway.  In reality, signaling pathways are composed of branching pathways and networks that fine-tune the response to a particular ligand.  However, understanding how activation of a specific signaling pathway by a specific ligand is complicated by a number of factors.  First, many receptors use the same limited number of second messenger systems.  This means that information specific to the ligand must be encoded into those second messenger systems. Second, receptor and second messenger activation occur rapidly but  cellular responses are slower. Thus interpretation of signaling dynamics, including where the signal is produced (location),  its temporal characteristics (how fast it is activated, how long it lasts, if it oscillate), and its intensity (how big or small is the signal?) all contribute to determining how cells will respond.