Postcolonial theory begins where anti-colonial politics leaves off. The occupations ended; the frameworks did not, and the question it asks is what remains of empire once the flags have changed.
Said's Orientalism (1978) is the founding text. Its argument is that "the Orient" is a European construction — an enormous, internally consistent body of scholarship, fiction and administration that produced the East as sensual, despotic, static and in need of rule, and that this discourse was not incidental to imperial power but part of its equipment. Said took Foucault's account of knowledge and power and applied it to a body of learning that had understood itself as disinterested.
Spivak pressed the harder question. In "Can the Subaltern Speak?" she argued that the very act of representing the oppressed — including by sympathetic Western intellectuals — reinscribes the relation it means to undo, since the subaltern is heard only in a vocabulary supplied by others. Her answer is closer to no than to yes, and the essay is as much a critique of Western radicalism as of Western conservatism.
Bhabha complicated the picture from the other side, arguing that colonial relations are never a clean imposition on a passive subject: mimicry produces colonial subjects who are "almost the same, but not quite", and that gap is destabilizing rather than merely humiliating. Hybridity, ambivalence and the third space are his terms for a colonial encounter that changes both parties.
The tradition's standing criticism is that it purchased sophistication at the cost of politics — that a discourse analysis conducted in expensive universities is a poor successor to the movements that actually ended colonial rule. Fanon, who belongs to both, is the measure of the distance.