Stands for "Resilient File System" and will be implemented in Windows 8 products only. It's basically been built from the ground up and will help users have more flexible control over larger storage volumes. At the same time, the new system will be more "resilient" against data corruption.
Windows 8 BlogReFS inherits the features and semantics from NTFS including BitLocker encryption, access-control lists for security, USN journal, change notifications, symbolic links, junction points, mount points, reparse points, volume snapshots, file IDs, and oplocks.
Sounds great but M$ has also stated that this new file system cannot be used with removable media (SD cards, USB Flash etc) or boot drives lol. So basically it would just be for really large storage volumes. There isn't a whole lot of info on performance numbers or other compatibility yet... thoughts?
As a side note, I think the first official Win 8 beta is due next month. so perhaps the new file system will be available for a test drive