When it comes to English - the world's lingua franca - there is such a wealth and depth of vocabulary borrowed from hundreds of sources, and grammatical rules (along with their exceptions). This results in a lot of arguments about what is strictly 'right' and 'wrong'.
And so there are two schools of thought: prescriptive and descriptive linguistics.
Prescriptive linguists believe that the rules should be obeyed, and try to enforce standardization for how the language should be employed. This includes spelling, syntax, grammar, as well as deciding the political and social correctness of certain words or phrases. Prescription tends to resist change in favour of keeping everything nice and regulated.
Descriptivism contrasts with prescriptivism in that it works on the basis of how the language is used, rather than how it ought to be used. Linguistic study must by definition be descriptive - how can you study a language without knowing how it's actually used? However, most language learning text books you'll find are very prescriptive in the way they explain how to use words and phrases, often with little room to manoeuvre.
In reality, for a language as idiomatic, varied and constantly evolving as English, the two should be able to work together, and indeed they do - but often their priorities and aims are different. Without a governing body to regulate the English language (such as the French have with the Academie Francaise), the argument of whether certain words or phrases should be formally 'accepted' will forever be part of our language.