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Knowing how to read your sparkplug will give you insight on how your engine is running, a quick look at a set of plugs can indicate sign's of poor tuning (unhealthy air/fuel mixtures) and to a degree the condition of the engine.

Heres are breif listing on what your plugs could look like and what the cause could be:

Normal: Ceramic will have a grayish-golden brown colour, electrode should look newish and there shouldnt be any signs of oil.

This is pretty much what your after, both the sparkplug and fuel-mixtures are functioning correctly.

Dry: The electrode, ceramic insulator, and inside metal surroundings contain black, sooty deposits

Sooty buildup is often caused by an excessively rich air-fuel mixture, this could be caused by either overfueling or a sparkplug that is too cold for the engines operating conditions. Your ignition system could be having problems creating a strong enough spark to ignite the mixture causing the engine to misfire or performs poorly, it may be getting choked out not fireup at all.

A leaner adjustment may be required by adjusting your AFM perhaps. You need to clean or replace your spark plugs and check heat range.

Overheated: Melted Electrode, ceramic insulator may be burned/discoloured

If its anything like this then theres a good chance your running lean or you have the wrong plugs installed. Getting a too hot plug will result in heat not being evactuated from the combustion chamber quick enough. Other possibilities include overly advanced timing or perhaps an incorrectly gapped and/or torqued plug.

Ensure that the cooling system of the vehicle is functioning properly, a cooler plug may be needed.

Oil-Fouled: Shiny, black and wet with oil

The Problem Poor performance, skips, hard starting, and sometimes a bluish smoke coming from the exhaust is present. Oil is getting into the combustion chamber. In a new engine, the rings may not have seated; in an older engine, ring and valve seals may be worn. Rebuild the engine.

Detonation: In the beginning, brown specs are noticeable on the ceramic insulator. At a more advanced state, you will see cracks and/or chips on the ceramic insulator.

A condition that every tuner should look out for. Engines timing could be advanced too far, poor quality fuel or fuel's octane rating is too low for your engines state of tune. Fuel timing or fuel to air ratio may very well be off, change plugs a get your car to a dyno.

Excessive Deposits: A red/brown or white powder coating on the ceramic insulator.

These deposits can come from additives in fuel and oil or be the result of oil leakage into the combustion chamber. Clean the plug and change the fuel to prevent deposits from re-occurring.