Along with its new operating system (macOS High Sierra), Apple rolled out an entirely new file system called Apple File System (APFS). Apple's programmers designed this new file system to operate on all Apple devices, not just desktops and laptops but iPads and iPhones as well. It addresses key issues that have been plaguing the old Hierarchical File System Plus (HFS+), which Apple has used for Macintosh computers since 1998. Its predecessor, HFS, debuted alongside Apple's first hard disk drive for Macintosh in 1985. Namely, APFS addresses file checksum, nanosecond time stamps, volume snapshots, and other features which were lacking under HFS+.
Additionally, APFS' primary optimization is for flash storage to increase read/write times on all their devices. With SSDs in most Apple computers and all mobile devices running on flash memory, the future of data storage is solid state, and now Apple has a file system that takes full advantage of that new reality. This was demonstrated during the Apple Worldwide Developers Conference in 2017 debuting APFS by comparing file transfer speeds against HFS+.
One of the coolest things about APFS is the way your computer upgrades to it. Instead of requiring a complete reformat to create the new file system while installing the new OS, in the process of upgrading to High Sierra, the file system upgrade happens completely in the background and preserves all of your files exactly as you left them. This is a bit like replacing every part of a car one-by-one, while it's still running, without losing anything in the glove compartment. It's an impressive feat of programming.
This is a bit like replacing every part of a car one-by-one, while it's still running, without losing anything in the glove compartment.