CFast memory cards, also known as CompactFast, are an ultra-fast variant of the popular CompactFlash (CF) memory cards. Rather than the Parallel ATA bus used in CompactFlash cards, CFast cards use the faster Serial ATA (SATA) bus, allowing for data transfer speeds up to 300 Mb/s.
Physically, CFast cards are the same size as CompactFlash, but they are not backwards compatible – you cannot use a CFast card in a CompactFlash camera unless it says it supports CFast cards.
If you're shooting 4K video - this guide might be useful in helping you decide which memory card to use.
Due to their much faster data write speeds, CFast memory cards are used by many high-end high-resolution professional video and broadcast capable cameras and camcorders, as well as a few high-resolution stills cameras.