With a little help from the Google Chart API, I've added the ability to create QR code t-shirt, mugs and cards over at cogteeth.com.
QR codes are quite nifty little two-dimensional barcodes which can store a surpising amount of data in a small space. There are numerous readers available to decode them including mobile phone apps which enable you to decode them using your phone camera.
Any data can be stored in these codes but there are a number of standards or conventions recognised by the readers. For example, many will detect an encoded URL and automatically pull up the referenced web page. The documentation for the zxing reader gives a good summary of these conventions.