It is taken into consideration that literature and the literary theory are of great importance in understading the cultural product of a given nation and the literary theory has coped and developed in parallel with the changes that mostly characterized the human experience , rughly speaking . The twentieth century was the era of great upheaval and anxiety ; this upheaval was manfisted in different aspects of life , one of which is literaure.Consequently theories of criticism have developed , but the culmination was in the 60s withthe introducing of "deconstruction " by Jacque Derrida in France, and later we have come to know other theories which are deconsructionist in as far as the theme of the " centre- margine" is questioned. well, deconstruction is a literary theory of criticism that has reread and reestablished a new relation between the centre , represented in any Eurocentric idea, and the margin which might be represented by the " other" as either the black , the woman , the belittled , as it were.
Now, post colonialism is a theory of criticism that has started roughly speakin in the 70s of the twentieth century.
it is a theory of criticism that has emerged to address the body of literature and other disciplines that came to reconstruct the ''identity" and the image of the "self " after the departure of the colonial power .Actually post colonialism is multi- disciplinary because it is interrelated with other disciplines like gender studies, culture studies.
Post colonialism is a shift in study and an attempt to reread and rewrite, to point out and to correct what has been missed in the previous analytical readings. The term post colonialism is defined in the Encyclopedia of The contemporary Literary Theory as:

Post colonial theory is a term for a collection of theoretical strategies used to examine the culture (literature ,politics ,history and so forth) of former colonies of European Empire , and their relation to the rest of the world )
So ,despite the fact that there has been colonial expansion, at a certain time in the past, by Eastern nations over western ones, yet post colonialism is interested in the post colonial era of the previously colonized countries with the colonizer being "Europe" and the legacy of the European conquest of the other parts of the world. In this regard the literatures of the Third World, Canada, Australia, Malta, New Zealand, even the literature of the United States of America, are postcolonial literatures though many labels and names were given to this body of literature called post colonial literature such as Third World Literature, Commonwealth literature, New Literature written in English, Migrant Writings, Black Writing and Diasporic Writing.

Now, one cannot understand post colonialism without reading Orientalism by Edward Said. The book has appeared in 1978 and it was revolutionary in questioning the Western representations of the other parts of the world. But along with Said other critics have been called post colonial critics (Frantz Fanon, Albert Mimi, Giatri Spivak, Bill Ashcroft, Tiffin, Griffiths ,Homi Bhabha, Abdul Jan Mohamed, Aijaz Ahmad and others with the list continues to have newly added critics) .And some of the major voices and works of postcolonial literature include Salman Rushdie’s novel Midnight’s Children (1981), Chinua Achebe’s novel Things Fall Apart (1958), Frantz Fanon’s The Wretched of the Earth (1961), Isabelle Allende’s The House of the Spirits (1982), J. M. Coetzee’s Waiting for the Barbarians and Disgrace (1990) . But maybe the most important thing to be said here about those critics mentioned above that they have, each in his\her turn, added a model or an approach to this body of theory called post colonialism.