At the 2008 MySQL Conference and Expo, The Pythian Group gave away
EXPLAIN cheatsheets. They were very nice, printed in full color and laminated to ensure you can spill your coffee* on it and it will survive.
For those not at the conference, or those that want to make more, the file is downloadable as a 136Kb PDF at explain-diagram.pdf
* or tea, for those of us in the civilized world.