"Second, spell penetration. Spell penetration is negative resistance, plain and simple. If the target has 100 resistance and you have 20 spell penetration, his resistance is 80 against your spells. When I say "resistance" in a formula below I mean "effective resistance:" (actual resistance - spell penetration)..." "How much spell penetration do you need? Depends on how much resistance your target has. I don't think we have a standard assumption just yet, the way we have a standard assumption of 5% crit/dodge/parry/block chance for mobs, but the consensus seems to be leaning towards your average mob having something like 20-30 resistance, which means 20-30 spell penetration. But that's really not much better than a WAG. And of course in pvp resistances will be all over the board."
http://wow.allakhazam.com/db/guides.html?guide=1010
в двух словах - снижает resistance противника на кол-во penetration... Вроде так, если не прав, не бейте 
Добавлено (2007-09-06, 08.59.45)
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А Resilience как я понял снижает шанс того, что по тебе будет нанесен крит удар..
примеры: http://thottbot.com/s30319 http://thottbot.com/s14910
- Aybolit 70 (шланх) 
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