Industry refers to a group of companies operating within a particular field which share common characteristics.

Industry sectors refer to a large economic segment that contains multiple industries. The economy has traditionally been divided for purposes of study into sectors to enable analysis of how the various sectors of the economy are functioning.  In the U.S., industry sectors are often classified within 5 categories:

1. Primary (extraction and production of raw materials such as agriculture, forestry, fishing, mining).
2. Secondary (manufacturing and construction such as factories, construction).
3. Tertiary (services such as retail, banking, transportation, tourism, healthcare, education).
4. Quaternary (knowledge-based activities / intellectual development such as research, Information Technology, academic institutions, government, libraries).
5. Quinary (industries that have a major impact on society's organization and efficacy such as key decision-makers in major corporations and domestic businesses that help keep society functioning including executives in the government, science and academic fields, public services like fire and police departments, domestic endeavors like childcare and housekeeping).

The largest industry sectors are automotive; chemical; electronics; machinery; steel; aerospace; textile; and metalworking. Other large sectors based on their contributions to the national gross domestic product (GDP) include: real estate rental and leasing; utilities; state and local; administrative/waste management services; healthcare and social assistance; professional, scientific and technical services; educational services; management of companies and enterprises; durable goods and manufacturing; arts, entertainment and recreation; construction.