The "Fortune 100" can refer to two different lists -- the top 100 companies in the Fortune 500 list or the Fortune 100 Best Companies to Work For. The Fortune 500 ranks the largest corporations each year based on their gross income. The list includes both public and private companies, based on publicly available data.
Best Companies to Work For
The Fortune 100 Best Companies to Work For doesn't look at how much money the company makes but rather at how the company treats its employees. One-third of the ranking comes from the company's responses to the "Culture Audit," and two-thirds comes from surveys taken by the company's employees. Only companies that have been around for at least five years and have at least 1,000 employees are eligible to be ranked.