cheat sheet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A document, especially a sheet of paper, containing information, such as test answers, used for cheating.
• noun A document containing summarized information used for quick reference.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sheet of paper containing notes used to assist (with or without permission) on a test.
• noun idiomatic Any summary or quick reference used as a shortcut or reminder, a crib sheet.