In addition to providing a speedier, more compatible able Internet experience with Internet Explorer 11, there's been modifications to how the browser handles passwords.
First off, if you use a Windows 8.1 computer and a Windows Phone 8.1 smartphone, saved web credentials now sync between registered devices. Those with these devices have already experienced this, and many more will be able to take advantage of this feature when Windows Phone 8.1 officially releases (soon). This is a hugely valuable feature for those that switch back and forth between a computer or laptop and a smartphone.
Secondly, IE11 now pre-populates web page credential fields using your stored information, both for Windows 8.1 and Windows Phone 8.1. For this to work, the site has to meet certain security criteria, so you might notice it doesn't work for every web site. Security criteria includes the normal parameters such as SSL, valid certificate, no frames, and others. Microsoft contends that the new feature is secure, but that some web sites may not be developed with security in mind, so the feature will only work if it meets the stringent security principles.
You can manage stored passwords using the Windows 8.1 Credential Manager.