Stands for "Resilient File System."
ReFS is a file system developed by Microsoft. It was first introduced with Windows Server 2012 and is primarily used by file servers and data centers. Many of its features focus on protecting a disk's contents from data corruption while minimizing downtime. It also includes features that speed up the performance of virtual machines and multi-drive storage pools.
The benefits of using ReFS over NTFS or other file systems focus on resiliency against corruption, hence the name "Resilient File System." It includes several features that actively monitor files to detect and repair data corruption even while the disk is in use. ReFS creates checksums for file metadata (and optionally for file data itself) that can indicate when data is corrupted. When used in a Storage Spaces volume (a Windows feature that creates a multi-disk storage pool similar to a RAID array), it automatically uses its mirror and parity volumes to spot when one copy of a file no longer matches the others. It can then replace the corrupt copy with a fixed version.
Despite its improvements over NTFS, ReFS is not yet suitable as a computer's default file system. Many of its data resilience features require using Storage Spaces, which benefits large storage volumes for archiving data while providing no benefit for the operating system and installed applications. These data resilience features also come at a performance cost, so ReFS operates slower than NTFS. It also lacks several features from NTFS, including full-disk compression, extended attributes, and disk quotas. More importantly, ReFS does not yet support bootable or removable storage devices, so you cannot use it on your boot disk or any external drives.
ReFS is supported only by Windows (8.1 and later) and Windows Server (2012 and later). Linux, Unix, and macOS cannot mount a ReFS volume directly and must access a ReFS volume's files through a Windows file server over a network.