What is SDHC Class Speed Rating?

The SD Association has created and defined 3 speed classes to help you identify speed and performance capabilities/minimum requirements of SD/SDHC, miniSD/miniSDHC, and microSD/microSDHC cards and the host products. The SD Speed Class Ratings specify a minimum sustained write speed for SDHC/miniSDHC/microSDHC cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6 MB/s*).