Prescriptivism in linguistics is the idea that rules exist for the use of language, and good quality writing demands adherence to these rules. Sometimes, of course, prescriptivism is correct. There are important rules that govern language use, whose violation is an impediment to clear and effective communication. Other times, however, prescriptivists out themselves as rule fetishists. You can see this brand of prescriptivism at work wherever there are English teachers who insist, baselessly, that it is "wrong" or "ungrammatical" to end a sentence with a preposition; or that infinitives, like inseams, mustn't ever be split without embarrassment; and, finally--one of Orwell's bugbears--that the use of the passive voice must be minimized, if not eliminated. English teachers, and no one else, maintain and enforce these rules, though they manage to convince some English speakers to adhere to them.
None of these rules are defensible.[*] See here and here for the sad story of how we managed to get to the point where people are educated into ignorance about prepositions at the end of sentences; or note, with amusement, that Orwell himself used the passive voice more often than his peers. Split infinitives, arguably, create inelegant constructions, but surely no one misunderstood the voiceover introduction to Star Trek because it decided to boldly go. In what sense, then, is it "wrong?" Unfortunately, too many people, including otherwise intelligent men like George Orwell, readily attempt to enforce spurious rules of language. See this excellent post for an analysis of why people do this, and try, like I do, to let it go next time you are tempted to do it.
[*]If you thought I ought to have used the singular there, add that rule to the list.
