I've heard of one-dimensional and two-dimensional barcodes, but hadn't yet heard about four-dimensional ones.
This heise.de article (in German) talks about it, and has a clickable example.
The four dimensions are horizontal; vertical; colour; and time.
By adding colour and time, you can encode more information in a barcode than in a two-dimensional one such as DataMatrix, while still being able to decode them with, say, a mobile phone (as long as it has a camera that can capture not only still images but also short videos).
