Signal transduction is a way that
cells "talk" to one another.
Cells communicate to coordinate what they do. They might be a group of cells all specialized to do something specific, mating cells, or cells all about to divide. Just like people, cells can communicate when they're close together, which is called local signaling, or when they're far apart, which is done by hormones, but both kinds of communicating use chemical molecules that the cell secretes and other cells receive. (Brain cells have a special thing that reaches out to other nerve cells. The molecules they transmit are called neurotransmitters and the small space between the two cells is a synapse.)
Sometimes when the communication is local signaling, cells just have molecules stuck on their surfaces that connect to each other, or they have tunnels between them through which signaling molecules can move. Mostly, the cell makes the signaling chemical, then lets these chemicals out to diffuse through the fluid outside the cell to other cells nearby. Cells far away from each other send their chemicals (hormones) through the blood stream to cells in other parts of the body, or even to other bodies. Either way, the receiving cell gets a signal molecule from the sending cell.
Cell membranes are made of lipid molecules. Cells have proteins embedded in their membranes. These proteins do a lot of different things, but one of the things that they do is receive chemical signal molecules. They're called receptor proteins. The signal molecules fit right inside them like keys into locks or pieces into puzzles.
After the receptor proteins get the signal molecules, they're usually changed. Proteins are just long strings of amino acids folded up, so when something else fits into them, they change shape. When they change shape, they affect other molecules inside the cell, and that's called transduction. At the end of a chain of changing molecules, some molecule affects what the cell does, making a response. The whole process looks like this:
signal molecule --> receptor protein --> signal transduction pathway --> some sort of cellular response
There are different types of receptors. One kind is a G-protein-linked receptor, which changes so that another protein nearby, a G protein (it's called that because it has a molecule called GDP on it). GTP (which has three phosphates instead of two) replaces GDP, so that the G protein, which now has more energy, activates another enzyme. Another kind of receptor is a tyrosine-kinase receptor, which has two sides that come together when activated and energize other proteins nearby. One other kind of receptor is a ligand-gated ion-channel receptor, which is like a closed tunnel that opens when a signal molecule binds to it, letting ions from outside the cell go into the cell to make changes.
In the second part, the signal transduction pathway, sometimes a lot of different proteins make one long chain that passes along the message. Sometimes what happens is that when each protein is energized (by having ADP changed to ATP-P stands for phosphate in this and GTP; phosphates are what carries all this energy in cells) it energizes the next protein. Another kind of signal transduction pathway. ATP is turned into cAMP, which takes the message to wherever the response needs to be. Calcium and inositol triphosphates (just another molecule) also do this a lot. These molecules are called second messengers because they relay the message to their destinations.
Finally, there is some kind of response to the message. A cell might change its shape (the cellular skeleton does this), move, make new proteins, or change some part of its metabolism or cycle (such as entering into the next part of its mitotic or meiotic-cell multiplication-cycle). The cell responds, just as a person responds when he receives a communication from another person. And then all the cells are happy because they're all communicating and getting along and doing what they need to do.