One of the best ways to target users is to advertise to them when they are in the right context.  Maybe they’re searching or surfing the web for pink furry gorillas because they love pink furry gorillas.  As luck would have it, your game is full of pink furry gorillas so if you can get your ad in front of them, conversion is a cinch.  Keyword targeting is a critical input in most ad targeting schemes, so you need to have your own library of keywords that are tuned to your game and include words like pink, furry and gorilla.