It would prevent the saving of the username and the password.... assuming that the web developer did not save the username in a cooke.
I'm not sure how familair you are with HTML, so i apologize if i'm going over something that you may already be familiar with. A web developer can place any number of forms within a web page. Furthermore, they can place any number of fields (text boxes, check boxes, radio buttons, drop down lists, buttons) within a form. The autocomplete=off tag can be applied at the form level or to individual fields. when it is applied at the form level, it affects all items within the form.
Here's some microsoft info about Autocomple=off
http://msdn.microsoft.com/en-us/library/ms533032(VS.85).aspx Here's a good tutorial about Autocomeple:
http://www.geocities.com/technofundo/tech/web/ie_autocomplete.html