What is the difference between colonialism, anti-colonialism and post-colonialism?

Colonialism is the act of occupation by a foreign country.  England, for example, colonized the States (hence the first thirteen "colonies.") 

Anti-colonialism is the act of trying to oust the occupiers from the home nation.  Thomas Paine's pamphlet, Common Sense is a prime example of anti-colonialist writing.  A recent example in literature would include One Hundred Years of Solitude by Gabriel Garcia Marquez, whose characters cling to their land despite the brutality of the occupying force. 

Post-colonialism is the period after the occupying country has vacated and rule is returned to the indigenous people (well, unless you are a Native American, but that's another topic.)    India and the United States are countries that existed both as colonies and now are post-colonized countries.

American colonial literature examples include the sermons of Cotton Mather and the poetry of Anne Bradstreet.  Indian colonial literature, though this is a recent work, includes E.M. Forester's A Passage to India. 

Of course, colonization and post-colonial periods are not limited to America and India.  Virtually all nations have endured some sort of aggressive, occupying force.  As long as their are occupiers, their will always be anti-colonialist writing.