QR Code usages
QR Code itself is only a medium, way to represent a data - like a binary code, set of letters, music notation. It's only way of displaying data in machine-readable maner. QR Code docs itself does not specifies meaning of data eg. does not tell how to represent url, telephone number, Android Market link.
Because of that library takes as a parameter raw string, it's up to developer of code generator and scaner developers to agree over how speciffic data will be represented, and what action should be taken by scanner (navigate to web, add contact, initialize call).
Some ways are quite common, and - mayby not officialy speciffied - but defacto standards, supported by majority of software scaners. Here you finde some examples
Contact Card (VCard)
Important! Try to limit size of data - larger codes are hard to read. More: