Colonialism and imperialism alike involve the exploitation of distant lands and peoples. In that sense, they both involve domination and control. Behind such actions there is invariably an attitude of superiority towards those deemed less civilized or developed.
In the nineteenth century, which was the heyday of European colonialism and imperialism, white Europeans—almost without exception—believed that subject peoples were racially and culturally inferior. This perceived inferiority was used as justification for the kind of economic exploitation that became part and parcel of the colonial project.
Although the underlying attitudes behind colonialism and imperialism are more or less the same, in practice some differences can be observed. For one thing, it is possible to operate an empire without necessarily colonizing land. In India, for example, the British did not colonize the country; there were no discrete communities of British settlers living among the indigenous population, yet India was the most important part of the British Empire—"The Jewel in the Crown," as it was known.
In Ireland, on the other hand, it was a different story. There, the British settled in relatively large numbers, particularly in the north of the country. This ensured a Protestant ascendancy in Ireland that allowed the British to maintain effective control over the island for centuries.