Read and write speeds for SD memory cards are measured in classes. A higher class means a faster card, so you'll see Class 2, 4, 6 and 10. Stick with 4 or 6 for photos and 6 or 10 if you shoot video with your camera. If your camera supports Class UHS-I, that's an even faster card. For CompactFlash cards, you'll see ratings like 50x, 133x, 266x, 300x and 600x. Take the speed and multiply it by 150KB per second - so 133x would be about 20 megabytes per second. 266x is a good speed for most people. Keep in mind that some CompactFlash makers advertise read speed instead of write speed. Make sure you know how a card is measured before you buy