Imperialist countries take control of other countries. They may use military force to do this. However, they may also avoid using military force. They may simply establish economic and/or political control over the other country.
Imperialist policies of countries or kingdoms have goals of taking over more land to create bigger empires.
The ruler of an empire gains natural resources such as agricultural and mining products. Human resources are also gained. Conquered peoples may be forced into slavery or into low-wage work. They may also be called into military service on behalf of the empire. Trade markets are also gained. The people under an imperial government may be forced to buy products from that government or from that government's industries.
Thus, a policy of imperialism can vastly enrich the imperialist country. This benefit can extend over a long period of time.
Imperialism has been practiced throughout recorded history. However, there is one historical period that is specifically known as the Age of Imperialism. This period extends from the early 18th century to the mid-20th century.
During the Age of Imperialism, modern, industrialised countries competed for control of pre-industrial parts of the world. In pre-industrial areas, natural and human resources were widely and cheaply available. The imperialist countries that acquired the largest empires during the Age of Imperialism are The United Kingdom, France, Germany, Japan, and the United States.