Spellcraft is a mechanic introduced in Patch 23.2.0.137922. Minions with the Spellcraft keyword grant a temporary spell each turn. That spell can give your minions a buff that lasts until the start of the player's next Recruit phase.
Spellcraft effects only last until your next Battlegrounds/Recruit phase. That means that each turn you get to choose where you put your spells for the next Combat.
Spellcraft spells leave your hand at the end of your turn—if you don't use them.
Spellcraft spells are generated as soon as you play the Spellcraft minion, and at the start of each Recruit phase.
If your hand is full, Spellcraft spells will "wait" until there is a free space in your hand, instead of getting destroyed or being delayed a turn.
Golden Spellcraft minions will generate Golden versions of the Spellcraft spells, with double the effect.[1]