It looks intriguing. It comes in a bottle much like ink, put on using a brush... or I suppose other instuments might also be used. It comes in several colors. Can be watered down or not. Suppose to be smudge proof. It looks very intriguing, but I've said that already, haven't I.
If interested you can google it using 'Rewettable Derivan Liquid Pencil' (or perhaps even just liquid pencil. ??)
Here's a 10 minute youtube video showing someone using the liquid pencil. You don't have to watch the whole thing to get a feel for the 'tool'. It sure looks kewl.
(Its certainly possible that liquid pencil has been around for a long time, but I've only just stumbled across it.)