It is a format specification of a File system for storing files on hard disks by Apple Mac OS (Apple Macintosh) computers, but can also be found on read-only media such as CD-ROMs.
HFS was designed in the late 1980s as the native file system for Macintosh computers. It was necessary because Mac computers used richer data than other commonly available file systems, such as FAT, used by DOS and Windows, provided.
HFS+ (aka. HFS Extended) is an improved version of HFS, supporting larger files (64 bit length instead of 32 bit) and using Unicode (instead of Apple's own, limited, encoding) for naming the items (files, folders).
Both versions use B-trees to catalogue the filesystem.
In late 2002 Apple added optional journaling features to HFS+ for secured data reliability. These features were accessible in Mac OS X Server, but not the standard desktop client. However, in 2003 Mac OS X version 10.3 set all HFS+ volumes on all Macs to be journalled by default.
While HFS may be seen as a propriatary format, modern Operating systems provide software to access HFS formatted disks. See the MacWindows website for solutions. There is also software available for Unix and Linux systems, to at least read HFS, but perhaps not HFS+, formatted disks.
Technical information on the HFS formats is available from Apple's Technote 1150.