English has never procured an official Academy to regulate its use, as have other languages including French and Spanish, so you could argue it is much freer to evolve and reflect the cultures in which it is spoken. However, the choice of which words should be included in each new edition of the dictionary is taken by a few individuals. Their aim is to highlight words that have entered the language naturally over a period of a few years, rather than to artificially invent new ones. The OED explains the selection process as follows:
Finding new words
Oxford University Press has one of the largest and most wide-ranging language research programmes in the world. Our most important resources are the Oxford English Corpus and the Oxford Reading Programme. The Corpus consists of entire documents sourced largely from the World Wide Web, while the Reading Programme is an electronic collection of sentences or short extracts drawn from a huge variety of writing, from song lyrics and popular fiction to scientific journals. It's based on the contributions of an international network of readers who are on the lookout for instances of new words and meanings or other language changes.
Keeping track and making choices
We continually monitor the Corpus and the Reading Programme to track new words coming into the language: when we have evidence of a new term being used in a variety of different sources (not just by one writer) it becomes a candidate for inclusion in one of our dictionaries. For every new dictionary or online update we assess all the most recent terms that have emerged and select those which we judge to be the most significant or important and those which we think are likely to stand the test of time.
Evidence
In previous centuries dictionaries tended to contain lists of words that their writers thought might be useful, even if there was no evidence that anyone had ever actually used these words. This is not the case today. New terms have to be recorded in a print or online source before they can be considered: it's not enough just to hear them in conversation or on television, although we do analyse material from Internet message boards and TV scripts.
Timeline
It used to be the case that a new term had to be used over a period of two or three years before we could consider adding it to a print dictionary. In today's digital age, the situation has changed. New terms can achieve enormous currency with a wide audience in a much shorter space of time, and people expect to find these new 'high-profile' words in their dictionaries. This presents an additional challenge to lexicographers trying to assess whether a term is ephemeral or whether it will become a permanent feature of the language.
Personal inventions
People often send us words they have made up and ask if we will add their invented terms to one of our dictionaries. Unfortunately, the answer is usually no, because we only add words that we consider to have genuinely entered the language: we assess this by looking at all the evidence we have in our databases. Of course, some invented words do catch on and become an established part of English, either because they fill a gap or because they are describing something new. Examples of this type of invented word include wiki, quark, spoof, and hobbit.
http://oxforddictionaries.com (last accessed 17/01/2013)The complete list of words recently added to the OED can be found here. looking at many of the new additions, it does seem to me that (despite the OED's attempts to chose words which are likely to stand the test of time) many of them may become redundant as times change and cultural references become obscure. Below are some examples, together with their definitions:
