I've read the documentation and watched the WWDC APFS preso, so you don't have to. Here's what you need to know, based on what Apple has revealed so far.
Why a new file system?
File systems are absolutely critical and heavily used, and as a result they typically take years to harden. For Apple to write their own, they had to have a good reason.
That reason is that they wanted a single FS to run on all their devices, ranging from the high-powered Mac Pro to the tiny Apple Watch. That's good news for developers - iOS encryption, for example, isn't the same as on macOS.
APFS was architected with built-in encryption from the start. If you use File Vault on your Mac today, you give one password to the entire drive, so security is a blunt instrument.
With APFS you have choices: no encryption, one key per volume (like File Vault today) and multikey encryption - a password for metadata and other passwords per file and/or per extent.
You'll be able to really lock down critical information, but I hope there's some way to manage all those passwords.
Almost all Apple devices ship with flash-based drives today. Flash has very different characteristics than hard drives: writes are costly; reads are very fast; so you want to limit the former and take advantage of the latter.
Unlike most operating systems, whose I/O stacks are optimized for maximum bandwidth, Apple is optimizing APFS for low latency, to make all its devices as responsive as possible. Much fewer spinning beachballs waiting for file system locks to be released.
HFS+ can corrupt data if the system crashes during I/O. APFS implements a subsystem that makes all I/Os atomic: either an I/O completes or it doesn't; there are no partial I/Os that corrupt data.
Old apps that aren't being updated will still work with APFS because of HFS+ compatibility. This will also allow Apple to roll out APFS non-disruptively - or at least that's the plan.
Some of my favorite apps haven't been updated for years, so I'm all in with this feature.
One of the biggest changes will be to Time Machine. TM has a great UI, but the underlying technology makes it almost unusable for many pro users. For example, TM uses symlinks to relate backed up emails to individual email files. If you get and delete a lot of email - as most pros do - those symlinks are updated with every TM backup - a slow and costly process.
APFS implements snapshots - common in enterprise storage systems - using copy-on-write (COW) technology. The snapshot "freezes" your data a specific point in time and only copies the old data when it is written to in an update. Since only updated data is copied, snapshots require very little additional capacity, allowing them to be freely used.
This enables frequent "backups" at the directory or file level with much less overhead than Time Machine. It may also enable a feature called versioning, which creates a new version of a file with every save operation.
The roll out
APFS will become the default file system for all Apple devices next year. Apple will provide an in-place upgrade path: you won't have to backup and restore yor data.
The Storage Bits take
HFS+ has been functionally obsolete for at least a decade. With the rapid growth of large video files, Apple needed a modern file system to protect user's digital assets.
While APFS is definitely moving in the right direction, the quality of the implementation is key to the user experience. It's traditional to urge caution at major OS updates, and the APFS update will require more caution than most.
In the longer view though, APFS looks to be a solid foundation for the Apple ecosystem for the next 20 years. Stay tuned for more info as the beta matures.
Courteous comments welcome, of course.