(This is taken from the first day lecture notes for my "Video Game Level Design" course. The goal was to introduce level design as merely a subset of larger field of design, emphasizing that people have shaped the clothes / packaging / built environment around us and how level design reflects our world.)
What is design?
You hear the word thrown around a lot: fashion design, web design, interior design, graphic design, urban design, industrial design... and game design. What do all these fields have in common? They all focus on users, their behavior and how to influence that behavior -- whether it's to get more clicks or to reduce crime or to get you to buy another cheeseburger. People will use a thing depending on how we make it work.
At the supermarket, where are the daily staples -- stuff like milk and eggs -- usually found? In the back of the store, right? But to get to that delicious quart of low-fat white gold, you'll have to walk through another aisle -- oh, look, cereal! Cookies! Did I mention you brought your kid with you? Your kid wants some of that Killing Floor Krunch or Chocolate Zombies or whatever. It'd go great with your milk. By putting your goal in a distant area, supermarkets entice you with all this other crap and influence your purchasing and spending habits.

