It seems many people get confused about the differences (and similarities) between "colloquial" and "slang", so what exactly does each term apply to?
But to be even more thorough it seems to me we can also include a couple more terms which are often applied to language and arrange them into a scale from most acceptable to least acceptable: "informal" > "colloquial" > "slang" > "vulgar"
Are any two synonyms? Do they all overlap semantically or only with their closest neighbours? Or should some of them be seen as independently variable aspects of a word or utterance?
Finally, could each term apply equally as well to a word, a phrase, and a grammatical or syntactical usage?
EDIT
Perhaps I should've also included "nonstandard" – not sure whether it belongs to the left or the right of "slang" though.
Best Answer
[Edited, with examples:] There is some overlap between these terms. People will often even disagree whether a certain expression is best considered informal, or rather colloquial, etc. This is merely an attempt at cataloguing possible associations. If you have suggestions for improvement or refinement, do not hesitate.
Informal:
Colloquial:
Slang:
Vulgar:
Other relevant words:
Jargon: technical or academic language or terminology. This is usually perceived to be somewhat formal, and inaccessible to those outside the field in question.
Vernacular: refers to the native language of normal people, when a different language is used by some groups in society. So this is about different languages, not merely different registers or levels of formality. Usually, the non-vernacular language is used by the educated or higher classes, such as Latin until ca. the 19th century.
I will give a few examples, best description first: