HFS, also known as the Hierarchical File System, or Mac OS Standard
today, was the file system format used by Macintosh Computers.
The HFS format was used by Macintoshes on hard drives since the early systems; the floppies had a different FS. HFS gave each file both a resource and a data fork, storing media like icons and images and sounds separate from the data.
It was also case-insensitive, ie. a filename in all caps was the same as a filename in all lowercase. I don't know if it was due to the File System or the OS, but it didn't matter to the User. The file's path was also separated by the : character, instead of the / used by many other systems. HFS also stored creation dates as well as modification dates, and used creator and type information as metadata within the file.
HFS also supported Aliases, a type of Symbolic link, where moving the original file also moved the reference. On an HFS disk, files are limited in size to 2GB (due to the used of signed 32 bit integers to store file lengths). This was because HFS divides the disk into a maximum of 65,000 allocation blocks, no matter how big the disk. HFS stores the date in seconds since midnight, January 1st 1904, but uses local time.
HFS+, known as "Mac OS Extended," superseded this file format after Apple Computer released Mac OS 8.1. It had many improvements for the modern computing world, like permissions, support for larger hard drives past 2GB, etc. Apple tried to make the HFS+ format backwards compatible, and almost all programs have no problem handling it. HFS is the file format that wraps around the HFS+ format, so when you install a HFS+ hard drive in a computer and the OS can only read HFS, it won't show up as unrecognizable (which would then ask you to format it). Instead, you'll see an empty directory, with a single "Where_have_all_my_files_gone?" text file to explain what to do.
HFS is mostly unused now, it became outdated around 1999, but you could still format a drive to HFS using Disk Utility.app in Mac OS X, although the feature may have vanished from 10.4 onwards.