Stands for "Apple File System." APFS is a file system developed by Apple specifically for flash memory storage devices. It was released with iOS 10.3 in March, 2017 and for macOS 10.13 (High Sierra) in September, 2017.
The Apple File System is the successor to Apple's previous file system HFS+, which has been used for decades in Apple products. APFS is designed to provide a more efficient means of storing and accessing files on flash storage devices, such as smartphone and SSDs. It supports over 9 quintillion files on a single storage device, compared to the roughly 4.3 billion files supported by HFS+. APFS natively supports full disk encryption, while HFS+ does not.
APFS provides a number of significant performance improvements over HFS+. It requires less memory overhead and has lower latency than HFS+. This means reading and writing files is faster, speeding up common operations such as opening documents and browsing large numbers of files. Copying is much more efficient since file clones are created by simply adding a pointer to the original file rather than duplicating the file. This creates copies instantly without requiring additional disk space. When a copied file is modified, the updates are recorded as "deltas" or changes to the file.
Like its predecessor, the Apple File System supports TRIM to improve the lifespan of SSDs. It also adds "Space Sharing," which allows multiple APFS volumes to share the same free space on a physical storage device, or "container." This allows APFS-formatted volumes to grow (or shrink) as needed without repartitioning. Instead of journaling (implemented by HFS+), APFS tracks changes using a copy-on-write metadata scheme to record changes to the file system. This prevents file corruption caused by unexpected crashes and reduces the overhead of journaling.
NOTE: APFS is automatically used in Macs with flash memory drives running macOS 10.13 High Sierra or later. Updating a flash-based Mac to High Sierra will automatically update the file system from HFS+ to APFS. Macs with HDDs or Fusion Drives will not be updated to APFS.