For those people who werent playing the game back then, Junk is what we called Abzan before Khans of Tarkir made Abzan a thing
Like other permanent spells, you can only cast it during your main phase when the stack is empty, i.e., when you could normally cast a sorcery
Its a three colour deck, with Green, White and Blue being those represented
Finally, for four and one black you can bring back a creature from the dead under its owners control
Thats not to say that you wont find powerful cards in a Set Booster indeed, each pack of 12 cards (plus two bonus, non-game cards such as art cards) contains 1-5 cards at the Rare or higher rarity level, making them a more appealing product in some ways than, say, Draft Boosters
Theros: Beyond Death Collector Booster