Sorry for the sparse information, we are writing this as time allows.

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

Almost every single plug you purchase will have their gap set from the factory. It's rather rare that you will need to change this at all.

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

Initial gap sizes are determined by the plug manufacturer. Most standard design electrodes will have a gap between 0.6-1.1 mm (0.022-0.043 in), and this depends on a great number of factors. Consult the manufacturer's specifications and start with that gap.

As you increase the gap, you exponentially increase the amount of current required for the plug to generate spark. When the gap is increased, an ignition system upgrade is required to provide enough current for the plug to produce spark. So why increase the gap at all? To a certain extent, increasing the gap increases overall fuel ignition efficiency. A larger spark increases how much mixture ignites in a certain time period. However, at a certain point, this can cause spark blow out or the ignition event occurring too quickly and actually losing efficiency or causing too high pressure. Therefore it's important to start at the manufacturer recommended gap and not deviate too far from that.

All being said, perfecting the gap for maximum power is only worth about 0.1-1.0% increase in power over an "inappropriate" (but working) gap. So setting gap for longevity and consistency of operation is the best idea unless you are an F1 engineer.

As you increase cylinder compression, either through mechanical compression (pistons/comchams/stroke) or through forced induction (turbocharger/supercharger), spark blowout becomes an increasing possibility. Therefore it becomes necessary to decrease the electrode gap.

The two most popular plug manufacturers will be discussed below. Of the two, Denso is the far superior and recommended brand.

ALL Denso Iridium TT plugs have an initial gap of 1.0 mm (0.039 in). It should almost never be increased, though it can be decreased to as little as 0.6 mm. Other Denso plug gaps are: Iridium Power (0.7-1.1 mm), Iridium Racing (0.6-1.0 mm), Iridium Tough (0.8-1.1 mm), Iridium Plus (all are 0.8 mm).

NGK plugs generally have a gap of between 0.4 - 1.0 mm. NGK recommends increasing the gap by about 0.05-0.08 mm (0.002-0.005 in) for every 200 hp over 200 hp. However, it's also recommended to REDUCE that gap by about the same amount for every compression ratio increase of 2:1 or about 0.5 bar (7.7 psi) of added forced induction. As you can see, it is a delicate balance.

Heat Rating

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