At the top of the spark plug sits the connector, or terminal. This is where the spark plug wire attaches. The terminal connects inside the plug to the copper core of the center electrode, which is surrounded by insulation.
Next comes the hex head. This is where the socket wrench fits for tightening and loosening the plug in its hole in the engine. Just below this is a gasket that compresses tightly against the cylinder head. It's also known as a crush washer or the seat. Some plugs have a tapered seat, without an additonal seal. These are generally used in iron cylinder heads, while plugs with gaskets are usually found in aluminum cylinder heads.
The bottom half of the plug is threaded. This is the part that gets screwed, gently yet firmly, into place. A tiny bit of the center electrode juts out of the plug's lower end. And the whole thing is capped off with a ground electrode or ground strap. The spark that makes the engine run jumps the gap from the very end of the center electrode to the ground electrode. This is what ignites the air-fuel mixture that's been compressed by the piston.
The ground electrode is made of metal, with options ranging from stainless steel to titanium. It can come in several shapes as well, from notched or Y-shaped electrodes to triple electrodes with three little arms that seem to reach for the tip of the center electrode. As far as materials and shape of the ground electrode are concerned, you pretty much get what you pay for. High-end spark plugs made with exotic materials will cost more, but they'll also deliver better conductivity and spark.
Next, we'll learn what we all want to know: the basic rules for changing spark plugs.