Puzzle games are skill games that will make your brain smoking and will need a large thought to succeed.
When you play a puzzle game, you often have constraints. Sometimes, your character can not move as you would like, and you have to use your environment to reach the exit door. This is the case in games such as Demon Decimator.
There are also puzzles where you need to perform an action that seems impossible at first (even if it is perfectly feasible).