Later today (17/10/2013), Microsoft will formally announce Windows 8.1, its next major addition to the Windows PC operating system family. Current Windows 8 users are assured of a free upgrade to the operating system, but that won't happen today.
Microsoft announced that Windows 8 users can freely upgrade to Windows 8.1 starting October 18, 2013. The Upgrade Assistant tool that takes care of practically the entire upgrade process, should begin showing up on Windows Store for Windows 8 users, from the 18th. The users will need two conditions to be met - UAC must be enabled for Windows Store to function, and Windows Store must be logged in.
For everyone else, Windows 8.1 should begin selling from later today, priced at $120 for the base variant, and $200 for Windows 8.1 Pro. Windows 8.1 users will be able to upgrade to Windows 8.1 Pro by shelling out $99 for an upgrade pack.