Imperialism: A policy or ideology by which a nation extends its power by acquiring territories. It can be compared to business expansion where companies acquire smaller businesses to increase their market share.
Colonialism: The practice by which a powerful country directly controls less powerful countries and uses their resources to increase its own power and wealth. It's similar to outsourcing where companies use cheaper labor markets for profit maximization.
Decolonization: The process whereby colonies become independent of colonial rule. This is akin to employees breaking away from a corporation to start their own independent venture.