Puzzle Strike Shadows is the standalone expansion to Puzzle Strike. You can play it by itself or combine it with the base set. The expansion contains 10 new characters and 24 new puzzle chips.
Puzzle Strike is a card game played with cardboard chips instead of cards that simulates a puzzle video game such as Puzzle Fighter or Tetris. It's a deckbuilding game, which means you build your deck as you play. There are lots of deckbuilding games, but Puzzle Strike is the premiere competitive deckbuilding game. It's highly interactive due to the crash gem system, it's asymmetric with 10 characters to choose from in the base set, it gets to the action quickly because you start with character chips in your deck, and games tend to be exciting and close because whenever you're on the edge of losing, you're also able to do even more combos than usual.
Years of Puzzle Strike tournaments have shown that it's still interesting after even thousands of plays against the best players.