Meldezettel (Vienna)

Hello everybody,
I live in Vienna since one month, I am at the University for my PhD and I also teach in two courses in the University. Then I get a monthly salary, but my contract is only for 2 or 4 years. When I went to open a bank account (with Bank Austria) and when I was asking in some mobile shops about buying a austrian telephon number they told me that I have to show them the "Meldezettel". Now, can someone of you explain what is it? Is it a "residenza" or "domicilio"? What are the consequences in Italy (my country) if I do it? Is it possible NOT to do it? What about the soundness in Italy? Does something important change?
Thank you, I know there are a lot of questions but I really don't understand what Meldezettel is!
Rita