They were first created as an alternative to barcodes for tracking vehicle parts during production. The main advantages are they can be read at any angle by a camera rather than a scanner and can store any information rather than fixed.
But they can also be read by smartphones with the information being automatically stored in the phone - great for marketing your business, product or event. So they are displayed on business cards, posters, websites, magazines, etc.
To generate your own QR code, the easiest place to go is http://zxing.appspot.com/generator/ This uses the Google Chart program at http://code.google.com/apis/chart/docs/gallery/qr_codes.html
Create a code for your phone number, website or event. There is a full contact card but not all phones support it yet.
If your phone can't read the image, then try downloading one of these applications to your mobile phone.
and just for reference, qr code for my full contact details: