Imperialism: the policy of a government/country in which the country/government uses military power (as well as other forms of power) to influence other countries/territories. Imperialism is the "idea of overtaking others based on power to control others outside of their state."

Colonialism: the practice in which a power (government/country) establishes colonies or settlements outside its own borders for its own political and economic benefit. The colonizing nation seeks to use the materials and land in the colonies for its own good.

The main difference between imperialism and colonialism is that imperialism is the "idea" of controlling land in areas outside its own borders. Colonialism is the "practice" of act of overtaking another area in order to exploit the assets in the colony(ies.) (www.internationalrelations.org.)