He is one of my favorite authors and his book "Half-Real: Video Games between Real Rules and Fictional Worlds" has some excellent insights that I´ve used a lot in my classes in this year. Two quotes from this book that I want to highlight here are:
“To play a video game is therefore to interact with real rules while imagining a fictional world, and a video game is a set of rules as well as a fictional world”.
“These are the two things that video games are made of: real rules and fictional worlds”.
And I want to share an excellent diagram about the universe of the games from this book too:
And for last I want to leave a very interesting link from Jesper Juul´s site: its a "Dictionary of Video Game Theory". Click here to go to the URL.
JUUL, Jesper. Half-Real: Video Games between Real Rules and Fictional Worlds. USA: MIT Press, 2005.