Class refers to the speed of SD card.
Similar to CDs and DVDs, SD cards have certain rated write speeds.
Class 0 cards are those that were designed before the Speed Class Rating was defined by the SD Association
Class 2 SD cards have write speeds of 2 MB per second.
Class 4 is 4 Mbps.
Class 6 is 6 Mbps, and Class 10 is 10 Mbps and so on.
You may also notice cards with x speed ratings. Each x is equal to 1.2 Mbps. To get the overall speed, multiply the number that comes before x by 1.2 Mbps.
For example, 13x = 16.0 Mbps, 40x = 48.0 Mbps and 66x = 80 Mbps.
One thing to watch out for: the x-rating can refer to the read speed or the write speed. Almost always, the maximum read speed is faster than the maximum write speed. So, a card rated at 13x may have a read speed of 16.0 Mbps but only a write speed of 2 Mbps.
For this reason, paying attention to the Class speed rating is a more accurate way to judge SD cards. Speed Class Ratings are overseen by the SD Association, and manufacturers can only reference the Speed Class
Rating when talking about the minimum speed, whereas x speed ratings can refer to the maximum read or write speed.
Getting a fast card is important for camcorders and digital audio recorders, where audio or video is written in real time. It’s less important for things like cell phones and computers, where you’ll mostly be transferring data.