National and international statistical agencies use various industry-classification schemes to summarize economic conditions.
Securities analysts use such groupings to track common forces acting on groups of companies, to compare companies' performance to that of their peers, and to construct either specialized or diversified portfolios.
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.
They commonly divide economic activities into industries according to similar functions and markets and identify businesses producing related products.