Colonialism: "the control or governing influence of a nation over a 
dependent country, territory, or people." 
Imperialism: "	the policy of extending the rule or authority of an 
empire or nation over foreign countries, or of acquiring and holding 
colonies and dependencies." 
They are actually quite similar and each reinforces the other. The act 
of engaging in colonialism is an imperialistic act, but acts that are 
imperialistic are not always colonial. For example, (it's the only one I
 can think of, sorry if it's a bit contentious) some people define some 
of the America's current actions as imperialistic but they are not 
colonial. Hopefully that helps makes sense of things a little.Hope this helps!!