Khuzdul is the language of the Dwarves written with Cirth script. It appears to be structured, like the Semitic languages, around triconsonantal roots: kh-z-d, b-n-d, z-g-l. Not much is known of the language, as the Dwarves kept it to themselves, except for their battle-cry: Baruk Khazâd! Khazâd ai-mênu! meaning Axes of the Dwarves! The Dwarves are upon you!
Among the Languages, Khuzdul is unique in belonging to a separate language phylum, not related to the languages of Elves. Nevertheless there are many similarities between Khuzdul and the native tongues of men, such as Taliska, the language of the first and third houses of the Edain. This is because in the early days of Middle-Earth, before men crossed the mountains into Beleriand, they had contact to the Dwarves of the Blue Mountains and further East. Taliska was the ancestor of Adûnaic, the tongue of Númenor and the direct ancestor of the Common Speech, and both languages still had Khuzdul influences.
The Dwarvish language sounds much like Hebrew, and indeed Tolkien noted some similarities between Dwarves and Jews: both were "at once natives and aliens in their habitations, speaking the languages of the country, but with an accent due to their own private tongue…" (Letters, 176). 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.
It is said in The Silmarillion that Aulë, the creator of the first Dwarves, taught them "the language he had devised for them," which implies that Khuzdul is technically, in reality and fictionally, a constructed language.
For Peter Jackson's The Lord of the Rings, the linguist David Salo used what little is known of the Khuzdul to create enough of a language for use in the movies. This is usually referred to as neo-Khuzdul by Tolkienists.