The Age of Imperialism had a number of causes and effects on the industrialized nations and the rest of the world.
Some of the causes were rooted in the needs of industry. As Europe and Japan industrialized, they needed more natural resources and markets in order to keep their economies expanding. Nations competed to control Asia and Africa to secure their own economic success and political success.
Nationalism was also an important force as it motivated European nations to compete for colonial possessions. These powers used their economic, military, and political clout to force other countries to trade on Europe's terms.
Imperialism had many effects as it spread the economic, political, and social philosophies of Europe throughout the world. Christian missionaries were an important part of this process, as they won converts and established schools in many of the colonies.
Local economies were transformed by industrially-produced goods from Europe which flooded colonial markets and displaced traditional industries. As farmers grew cash crops for export, many people were pushed off the land and less food was grown for local consumption.
Imperialism took three major forms: colonies, protectorates, and spheres of influence. Here is a summary of European imperialist policy during the 1800s and early 1900s.
Africa
After paying little attention to the interior of Africa, by 1900 Europe had colonized most of Africa in order to secure land and important natural resources. All of the major Europeans had colonies and Britain controlled the highly strategic Suez Canal.
Asia
Britain continued to dominate India through the East India Company’s control of the Indian states. In China, after defeat in the Opium Wars, the major European powers and the United States forced convinced China to open its borders to trading and each established a sphere of influence in the dominated country. Meanwhile, in Japan, Matthew Perry of the United States forced Japan to start trading with the US. In response, Japan quickly transformed itself into a major industrial and imperialist power on par with the rest of the world powers.
Resistance
People in the dominated territories didn't accept European rule and superiority as a matter of fact. Resistance to imperialism took many forms, including armed conflict, like the Boxer Rebellion in China, and intellectual movements, like the foundation of the Indian National Congress in the mid-1800s. Initially, many of these resistence movements failed, but as time went on and the European powers were weakened by the world wars, many, if not most of these territories, became independent.