The Fortune 500 is a ranking of the top 500 American public corporations as measured by gross revenue, although eligible companies are any for which revenues are publicly available (which is a larger universe than “public companies,” as the term is commonly understood, meaning “companies having common stock that trades on a stock exchange”). Fortune magazine compiles and publishes the list annually.
Fortune 100 is a frequently used term used to reference the top 100 firms in this same list, and Fortune 1000 refers to the top 1000 firms ranked according to the same methodology.
The Fortune 500 should not be confused with the S&P 500, which is a stock market index.
All numbers are based on revenue figures for each company’s previous fiscal year, which may end on different dates for each company.
http://money.cnn.com/magazines/fortune/