Stands for "Hierarchical File System." HFS is the file system used for organizing files on a Macintosh hard disk. When a hard disk is formatted for a Macintosh computer, the hierarchical file system is used to create a directory that can expand as new files and folders are added to the disk. Since HFS is a Macintosh format, Windows computers cannot recognize HFS-formatted drives. Windows hard drives are typically formatted using WIN32 or NTFS file systems.
Since HFS was not originally designed to handle large hard disks, such as the 100GB+ hard disks that are common today, Apple introduced a updated file system called HFS+, or HFS Extended, with the release of Mac OS 8.1. HFS+ allows for smaller clusters or block sizes, which reduces the minimum size each file must take up. This means disk space can be used much more efficiently on large hard disks. Mac OS X uses the HFS+ format by default and also supports journaling, which makes it easier to recover data in case of a hard drive crash.