The new file system will be available for all of Apple's software platforms i.e. iOS, macOS (formerly OS X), tvOS, and watchOS, and applies for all Apple devices. On its developer page, Apple claims that the file system will put the spotlight on security, and make encryption its prime focus, with native support for encryption rather than via File Vault. It supports single-key encryption as well as multi-key encryption.
Apple summarises, "It is optimised for Flash/SSD storage and features strong encryption, copy-on-write metadata, space sharing, cloning for files and directories, snapshots, fast directory sizing, atomic safe-save primitives, and improved file system fundamentals."
It supports nearly all of the features of Apple's last file system HFS+. The predecessors HFS and the HFS+ are now almost 30 years old, and Apple certainly needed a new file system to meet the needs of the modern age.
While APFS is available to developers with macOS Sierra right now as a pre-release build, it's still not yet in full working order, and thus cannot be used as a startup volume. It cannot be used as a Time Machine volume, and users will not be able to apply File Vault encryption on it, or use it as Fusion Drive. APFS is currently also case-sensitive, though this is expected to changeby launch in 2017.
APFS upgrades to 64-bit inode numbers from the 32-bit file IDs supported by HFS+. There's also support for 1 nanosecond timestamp granularity. In comparison, HFS+ supports 1 second timestamp granularity. Also new is the ability to quickly make read-only Snapshots and editable Clones of directories and files. The full list of new features with APFS can be seen here.
At WWDC, Apple announced upgrades to watchOS, tvOS, the new iOS 10, and the new and improved macOS. iOS 10 brings in a host of improvements in the Photos, Maps, and Apple Music app. iMessages app also got an overhaul with many new features for making conversations better. Apple also opened up Siri and iMessages to the third-parties for better integration.