From Steve Sande, on OWC (Other World Computing)’s blog, Rocket Yard:
Without a file system, your Mac, iPhone, iPad or even your Apple Watch would not work. A file system is used by an operating system like macOS and iOS to read from and write to some sort of storage, whether it’s a huge hard disk drive or a speedy solid-state drive (SSD). All Macs currently use a file system called HFS+ (Hierarchical File System Plus), an outgrowth of the original Hierarchical File System that was introduced in 1985 with Mac System 2.1. With the roots of the file system now over 30 years old, Apple just announced a new file system called APFS (Apple File System) that will be shipped on all new Apple devices starting in 2017. As new versions of Apple operating systems roll out later in 2016 — macOS Sierra, iOS 10, tvOS 10 and watchOS 3 — existing devices will use APFS. (Note: You can read Apple’s APFS documentation here and watch an introduction video here.)
For the whole article, PYBT: Tech 101: Explaining the New Apple File System (APFS).
Graphic from the Rocket Yard post