What is a spark plug? Basically it's a high voltage bridge for electricity. When the electricity crosses the "bridge" (which is actually a gap between two contact points) inside your engine, the spark it makes ignites the gas vapors, which makes the engine go putt putt. How long they can do this without being replaced depends on lots of things. The condition of your engine, the purity of the fuel you're using, even your driving habits can affect the life of the plugs. But hey, they're cheap, so replacing them every so often can't be a waste of money. And while you're in there you can inspect your plug wires
In most cars, changing the spark plugs is simple and fast. Be sure to follow these direction in order, however, because a mix-up can be very confusing to fix. You'll see what I'm talking about in a minute.