Primary industries are those that harvest or extract raw materials from nature; they include agriculture, oil and gas extraction, logging and forestry, mining, fishing and trapping. In many countries, primary industries provide raw materials for manufacturing and heavy industries and are a stable economic base for rural communities, according to Statistics Canada. As a country develops, the percentage of its population working in the primary industries usually declines.
Secondary industries are the manufacturing and assembly industries, which use raw materials to manufacture finished products. Such industries include food processing and steel manufacture.
Tertiary industries are the service industries. Professionals who work in this sector include teachers, lawyers, and travel and real estate agents. The tourist, financial and computer industries are all part of this sector.
Quaternary industries are the newest; they involve the use of high technology to conduct research and development activities. An example of a quaternary industry is that of research into genetically modified crops.
The number of people working in the four sectors is often used to determine the employment structures of a country and establish how developed it is. In developing countries, most of the population works in the primary industries, while in developed countries, most of the population is employed in the tertiary sector.