What are the classes for SDHC cards and what do they mean?

Q: What are the classes for SDHC cards and what do they mean?

A: SDHC cards can typically have a Class 4, Class 6, or  Class10 designation. The class rating system is only relevant when shooting full 1080p HD video onto SDHC cards. It is not applicable to CompactFlash cards and is not relevant to still photography whatsoever.

By definition, class is the minimum sustained read/ write speed of an SDHC card expressed in megabytes per second (or MB/s). The class system was developed when flash based video came into vogue a few years ago, as a means to ensure the end result would be a drop-out free video when viewed on a television or PC. For example, your camera manual specs a Class 6 card for shooting 1080p video. This means you need a card with a minimum sustained write speed of 6MB/s (megabytes per second) to ensure proper video quality from your camera/camcorder.

In addition, the class rating is the minimum video recording speed of the card, not the maximum performance (speed) of the card