F2FS is a file system built to take into account the characteristics of NAND flash memory-based storage devices (such as solid-state disks, eMMC, and SD cards). While we’ve had support for F2FS for a while now, we recently added support for it as a rootfs and made it an option in the installer. Ext4 is still the default, but you can opt for F2FS if you select manual partitioning. For more features of F2FS, see the F2FS Wiki. Phoronix recently ran a comparison of Ext4 and F2FS, and you can read that report here.