Politically correct restrictions on what we can say and how we say it have been imposed by leftists on the American public to restrict debate and silence opposition. 
|“||...is the communal tyranny that erupted in the 1980s. It was a spontaneous declaration that particular ideas, expressions and behaviour, which were then legal, should be forbidden by law, and people who transgressed should be punished. (see Newspeak) It started with a few voices but grew in popularity until it became unwritten and written law within the community. With those who were publicly declared as being not politically correct becoming the object of persecution by the mob, if not prosecution by the state.||”|
-Philip Atkinson 
The modern politically correct movement began at the University of Wisconsin-Madison; one of the most liberal institutions in the United States and is often viewed as a liberalist degrading of the freedom of speech. George Orwell's Nineteen Eighty-Four famously incorporated the notion of limiting thought through language (see Newspeak). Words or actions that violate political correctness are called politically incorrect.
At American universities, liberals began imposing political correctness to prevent recognition of differences among gender, religion, belief system, sexual orientation and nationality. In the 1960s, feminists began to demand that the neutral pronouns he, him and his be replaced with expressions like "he or she", "him or her", "them", etc. They argued that no one would be able to understand that the masculine gender included the feminine gender in neutral contexts. But this was just part of their campaign to redefine the social roles traditionally associated with masculinity and femininity.
Political correctness or P.C. also means the alteration of ones choice of words in order to avoid either offending a group of people or reinforcing a stereotype considered to be disadvantageous to the group. More specifically, groups which (or whose putative "leaders" or other activists) claim some status as systemically oppressed or discriminated against 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.
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, Americans like Senate Republican leader Bob Dole 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, became offensive 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 question of politically correct language has spilled over from the use of racially descriptive words and affected the use of traditional language. In 1999, an aide to the mayor of Washington DC described a budget decision as "niggardly" (a word meaning "stingy," unrelated to the racial slur). The aide immediately came under criticism and was forced to resigned even though he had not said anything racially charged. However, his name was cleared within a matter of days and was offered to return to his previous position. 
As well as language, political correctness discourages the use of racial or stereotypes in fiction out of concern that these stereotypes may become self-perpetuating. For example, frequently seeing the image black gang-members decked out in gold chains, carrying guns and listening to rap may pressure young black people into seeing this lifestyle as the more 'acceptable' choice for their racial group. The common image of female-dominated occupations (nurses, secretaries, care workers, etc) and of male-dominated occupations (IT workers, military, machinery operators, mechanics, etc.) can discourage individuals of either gender from considering those occupations traditionally belonging to the other. Additionally films like "The Siege" and "True Lies" have been criticized by pro-Islamic groups as having Muslims portrayed as terrorists, despite the fact that most current terrorists are in fact Muslims. Thus political correctness becomes the consideration of all public statements and media for their unintentional social impact.
Political correctness can even affect terminology that's viewed by secularists as too "pro-religion" or an alleged "violation of the separation of church and state" in the United States. The best example of this is the active promotion of the use of C.E. and B.C.E. as the abbreviations used after dates (instead of the commonly and traditionally used A.D. and B.C.).
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 excluding 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 emphasize their difference from the "norm".
Some people 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 or to conform to stereotypical black behavior, 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 counseled against the troubles which will ensue as a result of anti-religious 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. Ravitch asserts that textbook producers are beholden to small non-elected educational boards in a few key states such as New York, Texas and California and that few citizens know anything about these boards or who holds the seats of power on these boards. It's not 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 from this corrupt system, while many elected Conservatives have rallied against this policy.
Presently, however, this may not be the case. In 2010, a group of conservatives on the Texas State Board of Education pushed the state to adopt educational standards that emphasized the role Christianity played in America's founding. Those educational standards also sought to counteract political correctness by removing parts of the curriculum that espoused political correctness and contradicted conservative values. As one of the largest states in the Union, Texas represents one of the largest markets for textbook publishers. 
Once a big state makes a textbook purchase, it's very difficult for a small state or any municipality 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 committees is sometimes done privately, to avoid politicizing textbooks, but the reverse has happened.  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.
Totalitarianism and Political Correctness
Political correctness can trace its origins back to the world of 1920's Germany, where Communist academics sought to impose their Marxist views on students. It is now acceptable in many Universities to have courses on gender, homosexual and African American studies, which, in fact, encourage the mainstream public to become different to avoid criticism.
- Atkinson; "The Origin and Nature of Political Correctness"
- Slouching Towards Gomorrah: 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