A game system, for those are curious, is a set of components that you can use to play a variety of different games. A deck conventional playing cards is my absolutely favorite example because everyone gets it. There are books upon books of different games you can play with just a deck of cards. There isn’t a game called Cards. There are families of games that you can play with those 52 little pieces of cardboard.
There’s a few places that it gets murky for me. One is when you have to add additional things to make the game work. I adore the Looney Pyramids but I also have to admit that a lot of the games involve adding more than just pyramids. Dice, playing cards, tokens, boards, Tarot cards, a Piece Pak set, etc. Although Looney Labs kind of killed that line of questioning by publishing the Pyramid Arcade and including all of that other stuff in the same box. And there are times when this kind of argument gets a little silly anyway. Poker needs poker chips or some equivalent (like actual money) to work so Poker doesn’t count SAID NO ONE EVER.
Yes, it is really nice when a game system is entirely encapsulated in one set of components. I mean, you have a game library in your pocket by putting a deck of cards in said pocket. But it is clearly too limiting to insist on that.
Another question you have to ask is if something is a game system or a game with a lot of variations. I remember being told that Quarriors was really a tool box because there were a variety of ways to play the game. I don’t think that makes it a game system since you’re still just playing Quarriors. Carcassonne having expansions doesn’t make it a game system. Just a game that can be expanded. On the other hand, Ablaze actually does cross into being a game system since the three rule sets that come in the box are fairly distinct. Ablaze is a very close call, though.
It’s also interesting when a game isn’t known for being made from a game system but clearly is. You can, of course, play checkers with nothing more than a Checkers set. But there are other, very solid games, that you can do that with. Lines of Actions and two-player Focus are my personal favorites.
And you can take the concept to work interesting extremes. You can argue that the early Cheapass Games, having you raid other games for components, turned your entire game collection into a game system :D