The SD Card Association has created and defined a SDHC Class Rating to help you identify speed and performance capabilities / minimum requirements of SDHC, mini SDHC and micro SDHC cards and the host products.

The SD Speed Class Ratings specify a MINIMUM sustained write speed for SDHC/mini SDHC/micro SDHC cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6 MB/s; Class 10: 10 MB/s; Class 16: 16 MB/s).

Please remember these are MINIMUM write speeds, many cards will write data a lot faster, such as the SanDisk Extreme III and Extreme Pro SDHC Memory cards.

It is with video use that the class ratings come more into play and are a more important factor when purchasing a memory card. Many camcorder manufacturers will designate a recommended "class rating" to ensure optimum video recording.