HFS Plus or HFS+ is a file system developed by Apple Inc. to replace their Hierarchical File System (HFS) as the primary file system used in Macintosh computers (or other systems running Mac OS). It is also one of the formats used by the iPod digital music player. HFS Plus is also referred to as Mac OS Extended (or, erroneously, "HFS Extended"), where its predecessor, HFS, is also referred to as Mac OS Standard (or, erroneously, as "HFS Standard"). During development, Apple referred to this filesystem with the codename Sequoia. HFS Plus permits filenames up to 255 UTF-16 characters in length, and n-forked files similar to NTFS, though almost no software takes advantage of forks other than the data fork and resource fork. HFS Plus also uses a full 32-bit allocation mapping table, rather than HFS's 16 bits. This was a serious limitation of HFS, meaning that no disk could support more than 65,536 allocation blocks under HFS. When disks were small, this was of little consequence, but as larger-capacity drives became available, it meant that the smallest amount of space that any file could occupy (a single allocation block) became excessively large, wasting significant amounts of space. For example, on a 1 GB disk, the allocation block size under HFS is 16 KB, so even a 1 byte file would take up 16 KB of disk space.
Like HFS, HFS Plus uses B*-trees to store most volume metadata.

History
HFS Plus volumes are divided into sectors (called logical blocks in HFS), that are usually 512 bytes in size. These sectors are then grouped together into allocation blocks which can contain one or more sectors; the number of allocation blocks depends on the total size of the volume. HFS Plus uses a larger value to address allocation blocks than HFS, 32 bits rather than 16 bits; this means it can access 4,294,967,296 (=2

Sectors 0 and 1 of the volume are HFS boot blocks. These are identical to the boot blocks in an HFS volume. They are part of the HFS wrapper.
Sector 2 contains the Volume Header equivalent to the Master Directory Block in an HFS volume. The Volume Header stores a wide variety of data about the volume itself, for example the size of allocation blocks, a timestamp that indicates when the volume was created or the location of other volume structures such as the Catalog File or Extent Overflow File. The Volume Header is always located in the same place.
The Allocation File which keeps track of which allocation blocks are free and which are in use. It is similar to the Volume Bitmap in HFS, each allocation block is represented by one bit. A zero means the block is free and a one means the block is in use. The main difference with the HFS Volume Bitmap, is that the Allocation File is stored as a regular file, it does not occupy a special reserved space near the beginning of the volume. The Allocation File can also change size and does not have to be stored contiguously within a volume.
The Catalog File is a B*-tree that contains records for all the files and directories stored in the volume. The HFS Plus Catalog File is very similar to the HFS Catalog File, the main differences being records are larger to allow more fields and to allow for those fields to be larger (for example to allow the longer 255-character unicode file names in HFS Plus). A record in the HFS Catalog File is 512 bytes in size, a record in the HFS Plus Catalog File is 4KB in Mac OS and 8KB in Mac OS X. Fields in HFS are of fixed size, in HFS Plus the size can vary depending on the actual size of the data they store.
The Extents Overflow File is another B*-tree that records the allocation blocks that are allocated to each file as extents. Each file record in the Catalog File is capable of recording eight extents for each fork of a file; once those are used extents are recorded in the Extents Overflow File. Bad blocks are also recorded as extents in the Extents Overflow File. The default size of an extent record in Mac OS is 1 KB and 4 KB in Mac OS X.
The Attributes File is a new B*-tree in HFS Plus that does not have a corresponding structure in HFS. The Attributes File can store three different types of 4 KB records: Inline Data Attribute records, Fork Data Attribute records and Extension Attribute records. Inline Data Attribute records store small attributes that can fit within the record itself. Fork Data Attribute records contain references to a maximum of eight extents that can hold larger attributes. Extension Attributes are used to extend a Fork Data Attribute record when its eight extent records are already used.
The Startup File is designed for non-Mac OS systems that don't have HFS or HFS Plus support. It is similar to the Boot Blocks of an HFS volume.
The second to last sector contains the Alternate Volume Header equivalent to the Alternate Master Directory Block of HFS.
The last sector in the volume is reserved for use by Apple. It is used during the computer manufacturing process.