Also known (incorrectly) as Indian ink, it is an ink made by suspending carbon particles, usually in the form of lampblack, in water and using either gum arabic or shellac (for waterproof ink) as a binder. Once waterproof Indian ink dries, it is usually permanent (much to my distress when I spill it).
Some types of indian ink do not soak into the paper when applied, which can come in handy when you want to fill in large areas with black ink.