
The SD Card Association (SDA) has placed a limit of 32GB on SDHC capacity, while technically speaking it could support up to 2 terabytes (TB) of storage if it could be developed. SDHC cards emerging onto the market created considerable consumer confusion as normal SD cards are used for many portable devices including digital cameras, camcorders, game systems, MP3 players etc., almost all of which didn't initially support SDHC cards. It was particularly confusing because 4GB SDHC cards were released when 4G SD cards were also being marketed.
SDHC cards are also graded by speed in three classes. Generally speaking, Class 2 offers 2 MB/s, Class 4 offers 4 MB/s and Class 6 offers 6 MB/s.
There are a variety of Secure Digital (SD) products available..
Secure Digital (SD) --> Original SD card offering capacities up to 8GB currently.
miniSD --> A smaller form-factor version of the original SD card.
microSD --> Currently the smallest flash memory card on the market.
miniSDHC --> High Capacity version of miniSD card.
microSDHC --> High Capacity version of microSD card.
SD plus --> A form of SD card developed by SanDisk.with USB additions.
Eye-Fi --> An SD card with built in Wifi capabilities of its own.
Gruvi --> A form of SD card developed for content distribution by SanDisk.