I have tried to google exactly this question, but couldn't find a simple explanation of that.
What are major differences(i.e. percentage)/pros of Extreme Edition compared to non-extreme edditions?
"extreme edition" parts generally have the highest clock speeds and core count for their product family. They are also unlocked for overclocking. Since intel split the high end and mainstream desktop parts onto different sockets extreme edition parts have always been on the high end socket (1366/2011/2011-3 etc).
The main downside is the price, extreme edition parts are very expensive and in some cases the gains over a much cheaper "k" part can be questionable. Especially if your application doesn't scale well to large numbers of cores.
In recent years the high end desktop platforms have also lagged behind the mainstream desktop platforms. So while high end parts can have more cores the parts at the top of the mainstream line often have faster individual core performance.