Imperialism is the act of a strong country taking control of a weaker country. The stronger country often instills it's values and culture onto the newly acquired country, which isn't always welcomed by the natives. This often leads to rebellion and unstable societies, due to the conflict of values.
A prime and common example of Imperialism is the several conquests and campaigns of Great Britain during the late 1700s, 1800s, and early 1900s. Some of the countries Great Britain took include the United States, Afghanistan, Canada, Australia, Sudan, New Zealand, Nigeria and several others. Great Britain was, by far, the most Imperialistic country in history. Just about every country Britain had is now sovereign. However. Britain still does control some lesser territories.
Japan led Imperialist campaigns from the mid 1800s to the early 1900s. They expanded into China and opened ports into Korea for trade. Japan attained resources from China and Korea, and other resources from Hokkaido and Okinawa.
Although the United States started as a British colony, it too was involved in Imperialism. The Hawaiian islands were annexed by the U.S. during the late 1800s. There were several petitions and movements against the idea, but the U.S. government went through with it. Today, Hawaii is considered to be one of our 50 states. Before these islands were annexed, Hawaii had it's own sovereign government, laws, and culture.
The Spanish Empire was one of the largest and most successful empires of all time. The vast majority of Southern America was colonized or taken over during Spanish Inquisitions. The Spanish once held territory in the land that is today the United States as well.