The rotation is an annual thing that Pokemon Company International does to keep the game fresh, new, and exciting. What they do is they take out a few of the older sets in the format, removing them from play. That means that, say, a Level Ball from Next Destinies is no longer legal to be played with the new format that takes place on September 3rd, 2014. Next Destinies, Dark Explorers, and Dragons Exalted (and I think Dragon Vault) have all been removed from play and any cards from them are illegal to use unless reprinted in a later set. For example, Darkrai EX from Dark Explorers can still be used, even though Dark Explorers has been rotated out. This is because Darkrai EX was reprinted for Legendary Treasures, which is still in the format. Major cards in an Yveltal EX deck are affected would probably be Sableye DEX, Dark Patch, and those are the only ones I can think of at the moment. Sorry. Wish I could have been of more help.
Hope this helps,
LoneWolf2113