An industry is classification that refers to a group of companies that are related in terms of their primary business activities. In modern economies, there are dozens of different industry classifications, which are typically grouped into larger categories called sectors.

Individual companies are generally classified into industries based on their largest sources of revenue. For example, an automobile manufacturer might have a small financing division that contributes 10% to overall revenues, but the company will still be universally classified as an auto maker for attribution purposes.

Source: Investopedia
http://www.investopedia.com/terms/i/industry.asp#ixzz48qlv6zSM