These two terms have a great deal in common, but also have real differences. Both of them refer to the practice of one country dominating another. In both, the dominant country has political and economic control of the country or region that it is dominating. In this sense, the two are very much the same thing.
The main difference is that colonialism generally refers to a situation in which a large number of people from the dominant country move to the dominated country. This was, for example, the situation with British colonization of what is now the United States. By contrast, when the United States took possession of the Philippines, it was much more of an example of imperialism because there was no large movement of Americans going to the Philippines to make their fortunes there.
Colonialism: A “policy” by which a country decides to “take, steal, etc.” things from another country or region of the world for the purpose of benefiting the country who created the policy.
Imperialism: The “carrying out” of the “colonial policy” of extending a nation's authority by “territorial acquisition” or by the establishment of economic and political “domination” over other nations.
Colonialism and imperialism means political and economic domination of one country to another. Simply colonialism as practice and imperialism is the idea driving the practice.
Colonialism is the implanting of settlements on a distant territory. Colonialism is where one nation assumes control over the other and imperialism is economic and political control,either formally or informally. Colonialism means a country conquers and rules over the region while imperialism means creating an empire and expanding into the neighbouring regions.
In Colonialism, one can see great movement of people to the new territory and living as permanent settlers. Imperialism is just exercising power over the conquered regions either through sovereignty or indirect mechanisms of control.