New "premium" features will be available in the free Windows 8.1 update that you will be able to download on October 17.
The free Windows 8.1 update will be available to download from 10pm on October 17 in Australia (12am October 18 if you're in New Zealand, or 4am October 17 in the US).
Windows 8.1 is a free update, and will be a free download from the Windows Store. You will also be able to buy new computers that have the update from October 18.
Some of the changes in Windows 8.1 include the ability to boot directly into a "desktop" mode similar to what you see in Windows 7, as well as bringing back the Start button, which is missing in Windows 8. On the Microsoft web site, a blog post describes it as bringing "many improvements"
There are various other changes, including a change to the way Windows syncs your files over the Internet to different devices, new apps, and changes to the way you can customise the Start Screen.
There are also some new features specifically for businesses, which we've summarised here