Novelty decks have a gimmick of some kind that pushes their sales. They may or may not be collectible - it's in the eye of the beholder. Things like the Square Deck, the Crooked Deck, the Round Deck all clearly fall into the gimmicky, novelty category. I have a pack of cards called "Cuss Words, European Edition" where every card lists a curse word in about a dozen different European languages - definitely a novelty pack. The clear-plastic playing cards of any brand, the glow-in-the-dark version by Kikkerland - definitely novelties. The Hoyle 2-pack "Slice" deck - seriously a novelty deck.
Novelty packs are meant to appeal to a broader audience due to having a "gee, whiz" factor to them, a curious feature that makes them stand out head and shoulders above the pack. If you see it on sale in Spencer's, there's a good chance it's a novelty deck.