At its most basic level, game design is presenting a player with carefully selected stimuli, with the intention of encouraging a response behavior that the player enjoys.
Game Mechanics: Any stimuli the game presents a player with, in the hopes of encouraging or discouraging certain gameplay.All games are made up of at least one mechanic; it's impossible to have a game without them.
A game mechanic that doesn't modify or reinforce player behavior is a bad mechanic, by this definition. A mechanic that intends to reinforce a particular behavior but ends up encouraging a different behavior is also a bad mechanic.
The standard game of Tag only has 2 mechanics:
- One player is "It"
- If the player who is It tags another player, that player is now "It"
Mechanics vs. Rules
Aren't these just the same thing? What's the point of using the fancier word?
All game rules are game mechanics, it's true. However, the reason I think the term mechanics is more useful here is because there are many other types of stimuli that I count as game mechanics, even though they'd never be considered rules.
For example, level design, audio, and UI can encourage certain player behavior, so they are important to utilize as game mechanics. When we only think in terms of rules, it's easy to forget the subtler game mechanics such as these. For that reason, I find it more useful to ignore the concept of rules entirely and only refer to mechanics.