Windows Phone 8.1 is an update to Windows Phone 8, it will bring great features. Leaked feature until now is the notification and control center, new revamped XBOX games hub, installing app to SD card, Cortana voice assistant, and many more.
@Nawzil8, Windows Phone 8.1 will be released to manufacturers in March 2014, then to developers in April 2014, and at last to users in May 2014. He also said that this is still leaked information, official release date will be announced by Microsoft at BUILD 2014.
Windows Phone 8.1 supposed to be available for all Windows Phone 8 device, just like another update like GDR update.
Some feature already leaked like Notification Center and Cortana in Action (video). Just stay tuned for another leak from trusted leaker :)