An Acronym for Unix FileSystem.
UFS started out as a layer on top of Berkeley Fast FileSystem (FFS), which was a derivative of the "FS" filesystem used in the first versions of UNIX at AT&T. It was introduced in the 4.4BSD distribution and is common in its derivatives, such FreeBSD, NetBSD and OpenBSD, as well as Solaris. The Linux Kernel only includes experimental support.
FreeBSD version 5.0 introduced UFS2, which allows FileSystem Snapshots, background fscking, as well as support for volumes over 1TB in size.
NetBSD (since 1.5), FreeBSD (since 4.0) and OpenBSD (since 2.3) all employ an addition to UFS called SoftUpdates. SoftUpdates is which is a mechanism that allows a file system to safely use delayed writes for metadata updates. The results are consistent filesystems after powerloss/crash and much improved performance.
See 
WikiPedia:Unix File System for more information.
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