Market share is the percentage of sales (by value or volume) that a brand or company has for a specific product or category.
Note: Market share calculations can range from specific products (i.e. Vanilla flavored ready to drink iced coffee) to extremely broad industries (i.e. food and beverage). Be sure you are aware of which industry segment(s) a report includes.
Above is an image from IBIS World (an industry database) that shows the market share of the Car and Automobile Manufacturing Industry report.
Web Resources
Use the resources through the Census Bureau's website to access information about businesses - such as the number of business that are in an industry (by NAICS code) in a specific area. Much of this information is also available via American Fact Finder.
A free resource that covers more than 400 industries. For each industry, it includes overviews, key issues, trends and the industry's outlook as well as financial bench marking, compensation surveys and valuation resources.
Deloitte offers industry prediction reports for state government, banking and securities, commercial real estate, energy and resources, insurance, payments, process and industrial products, renewable energy, retail and talent strategies industries
Yahoo Industry Center has a browsable Industry Index where you can find financial market data by industry, compare performance of top companies within an industry, and find news and top-ranked analysts.