From PackRat Recipe Wiki

A collection (commonly referred to as a "set," although that is technically a misnomer) refers to a grouping of cards that share a particular theme and collection icon (the little icon in the bottom left corner of any card). If you manage to vault at least one copy of every card in a particular collection, then you earn a "Feat of Wonder" for that collection.

• Neighborhood Barbecue is currently the smallest collection, weighing in at 10 cards.
  
• Monkey Lids is currently the largest collection, weighing in at 31 cards.