The second set in the mirrodin block introduces indestructable cards to the playing field. Also look for great rares like Arcbound Ravager and Darksteel Colossus.
The Darksteel set is everything you loved about Mirrodin and more: more equipment, more artifacts, more abilities, and even more cards. Darksteel is the first "small" Magic expansion to release with 165 cards. Among them, you'll find artifacts that are indestructible--which means just what it says: destruction effects don't send them to the graveyard. And if its an indestructible creature, damage won't do the job, either.
This set contains foil premium cards. These cards are identical to the normal cards in name, text, etc. They are just printed with a metalic foil finish. Every card in this set has a foil version. Foil versions of common cards appear in the common card slot in about one in 12 boosters. Foil versions of uncommon cards appear in an uncommon slot in about one in 20 boosters. Foil versions of rare cards appear in a rare slot in about one in 40 boosters.
Look for the following key cards:
Shield of Kaldra
Sword of Fire and Ice
Sword of Light and Shadow
Well of Lost Dreams