Per the newspost from Play!Pokemon, the 2013-2014 format will include only the following sets:
BW: Next Destinies
BW: Dark Explorers
BW: Dragons Exalted
BW: Dragon Vault
BW: Boundaries Crossed
BW: Plasma Storm
BW: Plasma Freeze
BW: Plasma Blast
BW: Legendary Treasures
XY Starter Sets
Black Star Promos numbered XY01 and higher (as of December 2, 2013, only XY01 through XY04 have been released).
Black Star Promos numbered BW33 and higher (and as of October 3, 2013, only Black Star Promos with "BW" in their collector's number may be used unless they were reprinted word-for-word in a currently tournament legal set).
2012 McDonald's Collection
And future sets will become play-legal as they are released.
Also, anything reprinted in these sets from an older set is play-legal. That means the following:
For Pokemon, it means it has to be the exact HP, Weakness, Resistance, Retreat Cost, Type, and have the same attacks (the attack text could be altered some by TPCi on a reprint to make it fit better with the way the rules on wordings work in the meta game) as a card in a currently legal set. For Trainers (now Trainer - Item)/Supporters (now Trainer - Supporter)/Stadiums (now Trainer - Stadium), it has to have the same name and the same effect when played. It doesn't have to be the same Trainer class as it was before, since the way that works is that the older cards are counted as the Trainer class that the reprint is, at least in most cases (for instance, the Trainer from the Base Set, Bill, was reprinted in HeartGold SoulSilver but had its card type changed to Trainer - Supporter, which is what all versions of the card must be treated as; on the flip side, the original Computer Search was banned entirely because of a card type change to Trainer - Ace Spec). Any changes in the card wording always go by the newer version of the card. For Energy, it has to be the same name and energy type (Special Energy versus Basic Energy, I mean), though you adopt any of the newer wordings.
There are a few exceptions to the older reprints being play-legal rule, including, but not limited to, the following cards:
Great Ball - Only the Emerging Powers and Boundaries Crossed prints are play legal due to a drastic change in the way the card functions compared to previous prints.
Computer Search - Only the Ace Spec print from Boundaries Crossed is play legal.