When Apple release the latest version of macOS (named Sierra) in Autumn, it will include the new APFS filesystem. Apple haven’t launched a new FS for a long time, and this filesystem won’t work for bootable disks at first. Eventually Apple will make this the default filesystem across the whole product line, from the iPhone, through to the Mac.
APFS Upgrade In Place
I have heard that Apple will release a tool to upgrade an existing HFS+ volume to APFS. If you run the upgrade I strongly advise you to make at least a couple of backups first. I would suggest a Time Machine backup and a bootable full disk copy, with online backup if you have it. While the upgrade is cleverly designed to minimise the risk of disk corruption, there is always the chance that failure could leave you with no option but wipe & start again. This is especially true of older Macs that you may have updated through multiple OS versions and accumulated all sorts of nasties along the way.
APFS Data Recovery
We are already working through the APFS documentation to keep ahead of these failures. We will have some recovery tools ready soon. As with most recoveries, the most important thing is to stop as soon as you hit a problem. Remember my new catchphrase, “Most damage to data happens AFTER the original failure!” Or, “It’s not the failure that killed the data, but the failed repair.” (I know, snappy right.)