The two terms I will apply to the Chinese colonization of Tibet are usurpation and dehumanization. The specific definition of the latter which I will use is the assumption of undeserved profit and privilege on the part of the colonizer and through the denial and destruction of the colonized; this usurpation is often accompanied by a falsified sense of entitlement, also called "legitimacy." The definition I will apply to dehumanization is taken from The Colonizer and the Colonized, by Albert Memmi; dehumanization is the denial of the colonized's humanity, which alienates the colonizer and reduces the colonized to no more than a function of his needs.
The most obvious way in which China has usurped is in the invasion and colonization of an already organized and governed territory whose leadership and people expressed opposition to becoming part of China. From there, Tibet's land was taken and used for commercial and industrial purposes, its natural resources and ecology harmed in irreversible ways, the temples and monasteries either destroyed or turned into tourist destinations that don't offer the former sanctuary and place of devotion to the Buddhist way, and millions of lives taken.
Further damage has been done through the rape and replacement of the Tibetan people by the Han Chinese population, many of whom were given employment or other compensation by the government of the Republic of China to go to Tibet. Not only are the Tibetan people being forcefully bred out through rape and watching their dwindling population be crushed and crowded out by the colonizers, they are no longer considered an ethnic group by Chinese officials and the land which is known as "Tibet" is only a fraction of what the peaceful country used to be. Along with the incidental destruction of their cultural identity and civil liberties such as the practice of Buddhism, many people -- mostly monks and nuns -- are jailed, tortured, humiliated in the most inhumane manner, raped and killed.
For what? Mostly for a political agenda, which is currently a question of legitimacy within China. Just as in Memmi's analysis of the colonial situation, the colonizer no longer believes in the communist agenda that was introduced to 20th century China, but still joins the mission in order to reap its benefits. As a matter of fact, in 2009 a classified cable that was leaked from the Beijing State Department provided strong evidence "that most Chinese youth are not joining the ruling Communist Party out of an ideological commitment but to seek better jobs and positions within the government." (http://www.centrepointnews.com/?p=12450) This is further evidence of the colonizer's illegitimacy and endless usurpation.
There is so much more that I'd like to include here, but I may never stop writing if I do, so instead I'll save something for my final paper.