What is the halachik source, if any, for lashon naki (clean speech)? Is there a portion of Talmud that deals with the specifics? On the surface, it seems to be employed inconsistently, by which I mean there are times where the torah or chazal will go out of their way to use euphemism instead of the proper noun or description (the below example from Pesachim is great). However, other times we find very graphic descriptions for instance, of female genitalia in shir hashirim, (albeit assumedly metaphorical) or tzoah rosachas (gitin 57a) with no compunction for 'lashon naki'.
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The Talmud discusses this issue in Pesachim 3a.
There it brings a number of instances where a verse uses extra letters to avoid saying a negative word. Here's one example that it brings:
The verse by Noach (Genesis 7:8) says to bring into the ark animals that are טהורה (pure) and animals that are אשר איננה טהורה (lit. that are not pure). This is instead of the shorter and more conventional טמאה (impure). I know this doesn't work out in translation so well, but in Hebrew it uses 8 extra letters to avoid saying 'impure'.
See there for more proofs and examples.
I think the usual source is Talmud Bavli Shabbos 33a, although the context there is aggadic rather than halachic. It identifies what is called "nivul peh" (obscene speech) as a cause for many a tragedy in the Jewish people and derives this from Yeshayahu 9:16. It also states there that for one who employs nivul peh, gehinom (hell) is "made deeper" for him.
Altogether, it would be difficult to identify this a a halachic prohibition , especially as I don't recall it being mentioned in halachic codes of law such as Mishna Torah or Shulchan Aruch. (Although Machzor Vitri seems to indicate that it is actaully forbidden.) It can certainly be said, though, that it is not a good thing.
With regard to what constitutes nivul peh, I would say that's more of a sociological question than a halachic one. Whatever is considered "improper language" in the society in which you live. (I suppose it would be similar to judging what is considered simlas isha and simlas gever with regard to cross-dressing.)
As a side note, the Taz (YD 124:1) writes that the above gemara about nivul peh is referring only to intentional speech. If one says something improper unintentionally, the above does not apply.