Political correctness or P.C. most commonly refers to the alteration of ones choice of words in order to avoid offending any group of people. More specifically, groups which (or whose putative "leaders" or other activists) claim some status as "systemically oppressed" will periodically attempt to change the terms by which they are referred to and demand that society as a whole change its usage of words as well. More generally, political correctness can refer to a broad range of lifestyle choices which fall into the realm of a political correctness movement aimed at changing (or expressing ones opposition to) Western culture as a whole.
An example of political correctness is the changing terminology used to described handicapped people. In the past the term "crippled" was perfectly acceptable and not considered offensive. At some point, somebody decided "crippled" was degrading and the preferred term changed to "handicapped". This, too, was eventually deemed offensive and "disabled" became the preferred term. Today, "disabled" is now considered degrading and "differently abled" and "physically challenged" are now the politically correct terms. The same can be said for the changing uses of terms for Black Americans: "Negro" and "colored", once perfectly acceptable terms, were declared degrading and offensive by some activists during the 1970s and "Afro-American" and "Black" came into use, which in turn gave way to "African-American", and in broader usage, "people of color".
The distinction for example between "colored people" (1950s-60s usage) and "people of color" (today) is an important one which illustrates the basis of political correctness.
Demands for politically correct language usage are rooted in the notion widely promoted among left-wing academics and sociologists that Western culture promotes systemic oppression against some groups by marginalizing them and disincluding them from the "norm"; the groups thus supposedly systemically marginalized are referred to as "the Other" by these left-wing academics. The implication is that these groups are systemically excluded from the mainstream. "Colored people" is therefore deemed offensive because the order of words puts "colored" first, emphasizing their difference from the mainstream, while "people of color" is acceptable because putting the term "people" first emphasizes that they are people and thus does not emphazise their difference from the "norm".
Reactionary politicians and pundits allege that instead of encouraging supposedly marginalized groups to integrate with and assimilate into the mainstream of Western culture, political correctness ironically encourages them to emphasize and indeed to wallow in their marginalization from society, and to make a public display of such. This is known as identity politics. According to this view, gays and lesbians are therefore encouraged to label themselves as "queer" and make public displays of "queerness" calculated to disturb the sensibilities of mainstream people, rather than integrate into the mainstream themselves; Black Americans are encouraged to adopt Afrocentrism and convert to Islam, etc.
The Language Police
Conservative scholar Robert Bork has charged that the educational system is a battleground where the future of America is being undermined and ill-served. He has counselled against the troubles which will ensue as a result of antireligious policies in the schools, permissive attitudes toward homosexuality and abortion, as well as welfare policies that have destroyed families since Lyndon B. Johnson's War on Poverty. 
In her book "The Language Police", Diane Ravitch documents just how easy it is to get a word, phrase or idea banned from modern textbooks and references. Textbook producers are beholden to small non-elected educational boards in a few key states such as New York or California. Few citizens know anything about these boards or who holds the seats of power on these boards. It's not a difficult for an interest group to mobilize a campaign to bombard the educational board. Meanwhile, the public is not even aware that their words or values are under attack.
Then, once a big state makes a textbook purchase, it's very difficult for a small state or any muncipality to make any changes. Thus, profound changes can be inserted into textbooks and reference books by putting pressure on a handful of educational administrators. The work of textbook selection committee's is done privately, to avoid politicizing textbooks, but the reverse has happened. It's easier to pressure a lawmaker who is not beholden to community standards, but instead is enamored with political correctness.  Ravitch has documented "bias guidelines" for major publishers of texts and tests. These "guidelines" consist of advice to writers and editors about words and topics that must be avoided. There are many outrageous examples of what is now considered objectionable in modern text books:
- "Women cannot be depicted as caregivers or doing household chores.
- "Men cannot be lawyers or doctors or plumbers. They must be nurturing helpmates.
- "Old people cannot be feeble or dependent; they must jog or repair the roof.
- "A story that is set in the mountains discriminates against students from flatlands.
- "Children cannot be shown as disobedient or in conflict with adults.
- "Cake cannot appear in a story because it is not nutritious. " 
Totalitarianism and political correctness
- The comprehensive and detailed control of all ideas, beliefs, and statements is one of the most problematic features of totalitarian regimes. 
- Slouching Towards Gommorrah: Modern Liberalism and American Decline. by Robert Bork published by Harper Collins (c) 1997
- The Language Police: How Pressure Groups Restrict What Children Learn by Diane Ravitch (c) 2003 published by Knopf