Market share is the percentage of the total market (or industry) sales made by one firm. Market share can be reflected as either a percentage of sales dollars, a percentage of units sold, or a percentage of customers. Percentage of sales dollars is the most common reference.
The market share for particular brands/products can be found in the database Gale: Business Insights. Once in the database, search by company or industry to locate the associated company or industry profile. Scroll to company/industry articles and select Market Share / Rankings.