Work Folders works hand-in-hand with Workplace Join, so if you haven't read about that yet, you can catch up on that here: IT Guide to Windows 8.1: Workplace Join. Work Folders is key technology, bringing SkyDrive-type (or Dropbox, or Box, Google Drive) of folder and file synchronization management integrated into the Windows 8.1 file system. So, instead of loading up a separate app, Windows 8.1 will just provide that ability inherently.
Another key aspect of Work Folders is that a device does not need to be domain-connected to enjoy the benefits of file and folder synchronization, again very similar to 3rdparty file synchronization applications. This also means that non-Windows devices could utilize Work Folders through the use of a 3rdparty app without needing to have Windows 8.1 installed, further enhancing the BYOD capabilities of Windows 8.1 and a Windows Server 2012 R2 environment.
A Work Folder Sync Share is created by IT on a Windows Server 2012 R2 installation (requirement) and full control over policies and access, including Rights Management, is retained at the administrative level to ensure security, privacy, and retain control over corporate data on personal devices.
One of the important things to remember about the Windows 8.1 release features is that much of the Enterprise capability built into the update is dependent on rolling out Windows Server 2012 R2. So, while it's easy to get excited about the features, don't forget that there's a whole other side of utilizing them which requires an even bigger deployment.
Nir Ben-Zvi and William Geraci provided a great overview for Work Folders at TechEd 2013:
This article is part of The IT Guide to Windows 8.1, a continuing series to make the case for Windows 8.1 in the organization.