Khuzdul was the language of the Dwarves, written in the 50-letter Cirth script (Runes). It appears to be structured, like real-world Semitic languages, around the triconsonantal roots: kh-z-d, b-n-d, z-g-l.
The language was created by the Vala Aulë, the creator of the first Dwarves, who taught them "the language he had devised for them," implying that Khuzdul is a constructed language even within context of the books. The language was rarely heard by outsiders and was a secret, and was thus not recorded by Elves or Men. The Lhammas tells that Khuzdul uniquely derives from the separate language family "Aulëan" (named after Aulë), separate from the "Oromëan" family that supersedes the Eldarin languages.
In the early days of Middle-earth, before Men entered Beleriand, they had contact with the Dwarves of the Blue Mountains and further East, and the Taliska language was devised, similar to the Dwarven tongue. Taliska would be the ancestor of Adûnaic, the later tongue of Númenor, and of the Common Speech.
Khuzdul sounds much like Hebrew, and indeed Tolkien noted that both Jews and his Dwarves were "once natives and aliens in their habitations, speaking the languages of the country, but with an accent due to their own private tongue". Another reason Hebrew was chosen as a basis for Khuzdul is that it is unlike any of the European languages, and thus sufficiently alien to western ears to show just how different the Dwarven speech was from the Elvish languages. Although Dwarven symbols are identical to those of Nordic Runes, the symbols that correlate to specific English letters have been mixed around and even a few Runes are inverted.
In the 2000's, linguist David Salo came up an expanded Dwarvish vocabulary, normally called neo-Khuzdul. For The Hobbit: The Desolation of Smaug, he fashioned Thorin's insult to Thranduil, îsh kakhfê ai-‘d-dûr-rugnul, meaning "May my excrement be poured upon the naked-jawed (unbearded Elves)", based off of Gimli's Extended Scene insult to Haldir in The Fellowship of the Ring, Ishkhaqwi ai durugnul, which merely meant "I spit on your grave".
The website "The Dwarrow Scholar", run by someone else, holds an entire fan-expanded Neo-Khuzdul lexicon that surfaced in 2012.