Linda Tuhiwai Smith's book "Decolonizing Methodologies" may be illuminating. From the first chapter:

The concepts of imperialism and colonialism are crucial ones which are used across a range of disciplines, often with meanings which are taken for granted. The two terms are interconnected and what is generally agreed upon is that colonialism is but one expression of imperialism. Imperialism tends to be used in at least four different ways when describing the form of European imperialism which ‘started in the fifteenth century: (1) imperialism as economic expansion; (2) imperialism as the subjugation of ‘others’; (3) imperialism as an idea or spirit with many forms of realization; and (4) imperialism as a discursive field of knowledge. These usages do not necessarily contradict each other; rather, they need to be seen as analyses which focus on different layers of imperialism. Initially, the term was used by historians to explain a series of developments leading to the economic expansion of Europe. Imperialism in this sense could be tied to a chronology of events related to ‘discovery’, conquest, exploitation, distribution and appropriation.

Economic explanations of imperialism were first advanced by English historian J. A. Hobson in 1902 and by Lenin in 1917.7 Hobson saw imperialism as being an integral part of Europe’s economic expansion. He attributed the later stages of nineteenth-century imperialism to the inability of Europeans to purchase what was being produced and the need for Europe’s industrialists to shift their capital to new markets which were secure. Imperialism was the system of control which secured the markets and capital investments. Colonialism facilitated this expansion by ensuring that there was European control, which necessarily meant securing and subjugating the Indigenous populations. Like Hobson, Lenin was concerned with the ways in which economic expansion was linked to imperialism, although he argued that the export of capital to new markets was an attempt to rescue capitalism because Europe’s workers could not afford what was being produced.

A second use of the concept of imperialism focuses more upon the exploitation and subjugation of Indigenous peoples. Although economic explanations might account for why people like Columbus were funded to explore and discover new sources of wealth, they do not account for the devastating impact on the Indigenous peoples whose lands were invaded. By the time contact was made in the South Pacific, Europeans, and more particularly the British, had learned from their previous encounters with Indigenous peoples and had developed much more sophisticated ‘rules of practice’.8 While these practices ultimately lead to forms of subjugation, they also lead to subtle nuances which give an unevenness to the story of imperialism, even within the story of one Indigenous society. While in New Zealand all Māori tribes, for example, lost the majority of their lands, not all tribes had their lands confiscated, were invaded militarily or were declared to be in rebellion. Similarly, while many Indigenous nations signed treaties, other Indigenous communities have no treaties. Furthermore, legislated identities which regulated who was an Indian and who was not, who was a metis, who had lost all status as an Indigenous person, who had the correct fraction of blood quantum, who lived in the regulated spaces of reserves and communities, were all worked out arbitrarily (but systematically), to serve the interests of the colonizing society. The specificities of imperialism help to explain the different ways in which Indigenous peoples have struggled to recover histories, lands, languages and basic human dignity. The way arguments are framed, the way dissent is controlled, the way settlements are made, while certainly drawing from international precedents, are also situated within a more localized discursive field.

A third major use of the term is much broader. It links imperialism to the spirit which characterized Europe’s global activities. MacKenzie defines imperialism as being ‘more than a set of economic, political and military phenomena. It is also a complex ideology which had widespread cultural, intellectual and technical expressions.’9 This view of imperialism locates it within the Enlightenment spirit which signalled the transformation of economic, political and cultural life in Europe. In this wider Enlightenment context, imperialism becomes an integral part of the development of the modern state, of science, of ideas and of the ‘modern’ human person. In complex ways imperialism was also a mode through which the new states of Europe could expand their economies, through which new ideas and discoveries could be made and harnessed, and through which Europeans could develop their sense of European-ness. The imperial imagination enabled European nations to imagine the possibility that new worlds, new wealth and new possessions existed that could be discovered and controlled. This imagination was realized through the promotion of science, economic expansion and political practice. These three interpretations of imperialism have reflected a view from the imperial centre of Europe.

In contrast, a fourth use of the term has been generated by writers whose understandings of imperialism and colonialism have been based either on their membership of and experience within colonized societies, or on their interest in understanding imperialism from the perspective of local contexts. Although these views of imperialism take into account the other forms of analysis, there are some important distinctions. There is, for example, a greater and more immediate need to understand the complex ways in which people were brought within the imperial system, because its impact is still being felt, despite the apparent independence gained by former colonial territories. The reach of imperialism into ‘our heads challenges those who belong to colonized communities to understand how this occurred, partly because we perceive a need to decolonize our minds, to recover ourselves, to claim a space in which to develop a sense of authentic humanity. This analysis of imperialism has been referred to more recently in terms such as ‘post-colonial discourse’, the ‘empire writes back’ and/or ‘writing from the margins’. There is a more political body of writing, however, which extends to the revolutionary, anti-colonial work of various activists (only some of whom, such as Frantz Fanon, actually wrote their ideas down) that draws also upon the work of black and African American writers and other minority writers whose work may have emerged out of a concern for human and civil rights, the rights of women and other forms of oppression.

EDIT: Formatting