You can check out the Denso website for more information. In a nutshell, they are a low resistance plug with a very fine tip on them. End result, it takes less voltage to fire the plug and the fine tip creates a concentrated spark. Supposedly, it offers more consistent and hotter spark than even a platinum plug. In turn, it may offer a little more horsepower, little better idle quality, etc. I have a set in my '01 Crown Victoria...replaced my platinums. $12 a plug X 8 plugs...expensive. I didn't notice a difference except for a very slight difference in idle quality. I'm not sure if it was worth all that money. I can think of more productive ways to spend $100.
Side note: I am not sure if these are recommended for "hot" aftermarket ignition systems like those offered by MSD, Jacobs, and the like. The directions on my Jacobs Pro-Street ignition specifically says to use a conventional copper-core plug because it will quickly burn the tips off a platinum or split-fire plug.
My 2 cents.