The cards found within Bronze, Silver, Gold, Platinum, Diamond, Mystery and Draft packs will be made up of whatever the current base set is. Base Sets are changed on a 6 month cycle and will contain an assortment of all cards previously released in the base sets or expansions. Note that some packs, such as Variety Packs or Promo Packs may contain cards that are not from the base set.
As well as the core set, packs may also contain cards from the current expansion. Each expansion is released in three 'previews', lasting approximately one month each. In the third month, the preview is limited to Draft Packs and Advanced Packs (not Gold Packs ), but also contains cards from the first two previews. After this, cards from expansion sets will not be available in normal packs unless they are rotated into the base set.
For more information see the State of the Game: September 2012.