Economic activities can be classified in a variety of ways. At the top level, they are often classified according to the three-sector theory into sectors: primary (extraction and agriculture), secondary (manufacturing), and tertiary (services). Some authors add quaternary (knowledge) or even quinary (culture and research) sectors. Over time, the fraction of a society's activities within each sector changes.
Below the economic sectors are more detailed classifications. They commonly divide economic activities into industries according to similar functions and markets and identify businesses producing related products.
Industries can also be identified by product, such as: construction industry, chemical industry, petroleum industry, automotive industry, electronic industry, power engineering and power manufacturing (such as gas or wind turbines), meatpacking industry, hospitality industry, food industry, fish industry, software industry, paper industry, entertainment industry, semiconductor industry, cultural industry, and poverty industry.