Electrode

There are a number of electrode (anode and cathode) designs available. From standard to TT (or twin-tip). TT plugs have a piece of electrode extruded from both the anode and the cathode. They generally produce a superior spark at a lower gap, thereby increasing overall efficiency of the ignition event. Their major downfalls would be increased cost (generally about 20-50% more expensive) and decreased longevity. As spark plugs fire, their anode and cathode erode. Having both the anode and cathode with a needle (or tip) means that they must be coated in expensive metals, such as platinum, and therefore reach a period of decreased operation with fewer ignition events.

Gap

Gap size depends on a few things, most notably electrode design and cylinder compression.

Heat Rating

Otherwise known as temperature, your spark plug's heat rating needs to be selected depending on your engine's operating temperature.