Lewis Pulsipher @ Gamasutra: "What do the systems of games amount to?
The goal here is not to list what the objective or victory condition is in a game, but to say "what the game amounts to" or "what is actually happening", "what is the player actually trying to do?"
The biggest problem with this list is whether to include the psychological or just the "physical". Poker is about bluffing, about reading the other player, yet what the game amounts to in each hand is a form of pattern-matching plus collection (of money). I think I'll leave the psychological out of this list, and stick to the systems.
At some point another problem is, what is a game? For example, I'd say most single-player video games are actually interactive puzzles, not games, but we call them games. Fortunately, the list below also applies to many if not all puzzles."