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. 

What is the different about colonialism and anti-colonialism?

Well, there are two kinds of colonialism, according to most historians:  The first where a more powerful country assumes control of the government and production of a smaller country, and the second where a country assumes control and brings a large number of settlers to live in and settle the new colony.  This of course can occur where there is an existing community or in the absence of one.

Anti-colonialism is based particularly on the idea that outside forces intervening in the affairs of an indiginous population or community is immoral and that it generally causes problems that take decades if not centuries to be resolved.  It has grown in power since the heavily colonial periods of the 18th and 19th and even early 20th century as parts of the world that were colonized have struggled to get out from under the shadow of various problems and damage created during the colonial rule.