btrfs is a B-tree based, copy-on-write filesystem for Linux. It's main features include snapshots, checksumming of data and filesystem level RAID support. It's similar to ZFS.
Btrfs tag should be used with all questions referring to usage of features specific to this filesystem:
- defragmentation
- resizing and addition and removal of block devices
- it's RAID implementation
- snapshot creation and usage
Also see:
zfs for a similar copy-on-write filesystem.
http://btrfs.wiki.kernel.org for detailed information about the filesystem