The Black Speech is one of the languages of Arda. The Black Speech was primarily spoken in Mordor, as it was created by Sauron with the intention of having it replace all languages his servants in Mordor used, as well as having it replace the different varieties or Orkish. There are two forms of the Black Speech: the "pure" form used by Sauron, his Nazgûl and the Olog-hai, as well as a "debased" form used by most of Sauron's servants.
Sauron was only partially successful in his attempts to have the Black Speech replace all other languages in Mordor (and eventually Middle-earth, had he been successful in conquering it). While most of Sauron's higher ranking servants (including his lieutenants) and the Uruk-hai learned the Black Speech, most Uruks were not able or willing to learn the language, nor did the groups of Men in regions Sauron had conquered. As Uruk often corrupted and debased languages they spoke, the Black Speech heavily influenced their vocabulary and grammar, but it quickly mutated into various Uruk dialects. Elves refuse to use the Black Speech, as it draws Sauron's attention.