Speed class rating on an SD card is the minimum transfer speed for that card in MB/sec.
The SD (Standard Definition) card is a memory card used in digital camcorders, cameras, PDAs, mobile phones and other portable devices. SD cards are designated often with a "speed class" rating; this rating is most important in camcorders and other video-recording devices.
Definition
An SD card's speed class rating refers to the minimum transfer speed for that card. For example, an SD card with a speed class rating of 2 has a minimum transfer speed of 2MB per second. This does not mean the transfer speed may not be faster at some points, only that this is the minimum speed guaranteed by the manufacturer.
Performance
It is not necessary to know all of the technical aspects of the SD speed class ratings to understand that the higher the speed class, the better performance you will get from your SD card. A video shot with an SD card with a speed class rating of 6 will be of better quality than a video shot with a speed class 2 SD card, although the two videos will take the same amount of memory.
Use
SD card speed class ratings are important if you are shooting video because shooting video requires a constant transfer of information. Often the manufacturer of your digital camcorder or other video-recording device will provide a recommended speed class rating; make sure are not using a card with a speed class lower than the recommended rating. If you are unsure of what the recommended SD speed class rating is for your product, check your manufacturer's website or the SD Association's website for more information.
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