I think it is just: store bytes and let the reader decide how to deal with them. Starts with "http", it's a URL. Starts with "BEGIN:VCARD", it's a vcard. Primitive but it works well enough I guess.

Just by the structure of the contents. Clients are supposed to parse the string contents, realize it's a URL, and open the browser for example.

The best guide to this I've seen is here: http://code.google.com/p/zxing/wiki/BarcodeContents

This link (http://www.swetake.com/qr/qr2_en.html) have a detailed explanation about how data are stored

by parsing the string