HFS Plus or HFS+ is also known as Mac OS Extended (Journaled). It is the file system used on all Mac OS 8.1 and later, including Mac OS X, since 1998. HFS+ is an upgrade of HFS (HFS Standard or Hierarchical File System). And it continued as the default Mac OS X file system until APFS replaced it.
The "journaled" feature on this file system refers to a way of storing media on the disk. With this feature, files won't be corrupted when some accidents happen. For example, you unplug the drive while it's working, or your Mac loses power while the drive is in use.
APFS, short for Apple File System, was introduced with the launch of macOS 10.13 (High Sierra) in 2017. Apple has also designed it for optimizing access to solid-state drives (SSDs) and flash storage devices. APFS has become the default file system of Mac hard drives (or startup disks) for macOS 10.13 and later macOS releases.
APFS volumes also enable snapshots for creating a point-in-time and read-only instance of the file system, which can be a backup of your data. For Macs with SSDs before 2016, they have auto-converted to APFS when upgrading the macOS.
For the difference between HFS + and APFS, you can have a quick look over here:
1. Accodring to Malc's speed test, HFS+ is less stable than APFS and has slower read/write speeds.
2. HFS+ outperforms Time Machine backup compatibility. No matter what macOS you're using, you can save all Time Machine backups to HFS+ drives. While APFS is one available for macOS Big Sur and later systems to save Time Machine backups. If you want to use APFS formatted drive for Time Machine backup on macOS Catalina or previous releases, you need to convert APFS to HFS/HFS+ first.
3. HFS+ only allows file or folder encryption while APFS supports full disk encryption. HFS+ needs encryption added as a layer on top of the file systems via Core Storage while APFS has built-in encryption support .
For more details about HFS+ and APFS, read the following articles: