Answer

Companies can be screened by various data points.  Generally financial data points including revenue, sales and market cap are used to determine similar companies and or their competitors.

Go to the Business Library's Company & Financial Information section for links to useful resources including links to the following suggested resources:

Business Insights Essentials:  Provides Comparison Charts for companies where you choose the companies to compare. Or choose a company and review the tear sheet data for similar companies provided.

Factiva: Go to the Companies tab to search a company.  In the results choose Peer Comparison for selected financial variables. 

Hoover's: Allows you to build a list using various criteria and it has a competitor tab as part of a profile.

Mergent Online:  Provides a competitor's list of U.S., global or private companies. The list can be sorted by revenues, market cap and other variables.