The days when you changed your spark plugs every couple of years has ended. Back in the day, spark plugs really did wear out that often. A couple of things are different now. First, spark plugs are made of better materials that last longer and they're designed better. The second reason that plugs used to have to be changed was that they were fouled up with carbon deposits. The deposits built up when fuel wasn't burned completely. With modern engine management controls that just doesn't happen as often.
Engine control computers precisely time when fuel is injected into the engine and when spark plugs fire. Unless something's wrong, spark plugs just don't foul like they used to.
Electricity from the battery goes into a coil that allows power to build up to anywhere from 12,000 to 45,000 volts, depending on the vehicle. The engine management computer tells the coil when to release the power to the spark plug. The electricity travels through a wire from the coil to the spark plug. At the tip of the plug, a spark jumps between two electrodes and ignites the gas in the combustion chamber.
Some engines have more than one coil. Coils wear out and need to be replaced occasionally. Also, spark plug wires can wear out and need to be replaced.
Modern engines are delivering more power and better fuel economy all the time. That's largely credited to fast engine control computers, advanced sensors, electronic ignition and improvements to the lowly spark plug.
It'll be interesting to see where future developments take us. One last thought: it's critically important to have the right kind of spark plug for your vehicle. Because engines are designed to run with different internal temperatures, spark plugs have different designs to work properly within those temperatures. Your service advisor will be able to get the right plugs for your vehicle. And he'll be able to advise you on when you should replace your spark plugs as well.