SDHC
SDHC (Secure Digital High Capacity, SD 2.0) is an extension of the SD standard that appeared in June 2006. SDHC allows standard-compliant capacities in excess of 2 GB. SDHC cards are often formatted with the FAT32 file system. It uses the same form factor as SD, but the SD 2.0 standard in SDHC uses a different memory addressing method (sector addressing vs byte addressing), thus theoretically reaching a maximum capacity of up to 2 TB (2048 GB). However the SD Card association have artificially defined the maximum limit of SDHC capacity to 32 GB. SDHC cards only work in SDHC compatible devices, but standard SD cards work in both SD and SDHC devices. The SDHC trademark is licensed to ensure compatibility.
SDHC cards have SD Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":
Class 2: 2 MB/s
Class 4: 4 MB/s
Class 6: 6 MB/s
SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. See section Speeds above for a further explanation. One critical difference between the Speed Class and the maximum speed ratings is the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required. "Maximum speed" ratings are quoted by the manufacturers but unverified by any independent evaluation process.