The Official UK Taxi Firm Register

The black and white boxes are known as QR Codes and they come to us from Japan where they are very common. QR is short for Quick Response (they can be read quickly by a cell phone). They are used to take a piece of information from a transitory media and put it in to your cell phone. QR Codes can now be seen everywhere including magazine adverts, billboards, web pages or even on someone's t-shirt. Once it has been scanned by your cell phone, it will give you details about the business, such as Name, Address and Telephone Number.

The reason why they are more useful than a standard barcode is that they can store (and digitally present) much more data. The other key feature of QR Codes is that instead of requiring a chunky hand-held scanner to scan them, many modern cell phones can scan them. The full Wikipedia description is here.