SDHC (Secure Digital High Capacity) is an extension of the SD (Secure Digital) card standard which increases the flash memory card's storage capacity up to 32 GB. The SD format itself is based on the older MMC format. (see SD Card vs MMC)
SD cards used byte addressing, which limited the amount of memory it could support to 4GB. The newer SDHC standard uses sector or word addressing, which increases addressable storage to theoretically support capacities up to 2 TB (2048 GB). The current SDHC standard limits the maximum capacity of an SDHC card to 32 GB but it is expected that the specification will be revised to allow card capacities greater than 32 GB. In early 2010, SanDisk started offering a 64GB SDHC card.
SDHC cards have the same size and electrical form factor as older SD cards, so SDHC-devices also support SD cards. However, the reverse is not true. An SD card reading device will not support the newer SDHC standard. Look for SD/SDHC logo on the package/manual for host compatible devices such as digital cameras, digital video camcorders, and mobile phones.
edit File System
SDHC cards are typically formatted as FAT32 while SD cards are typically formatted with the FAT16 file system. However, both types of cards can support other general-purpose file systems, such as ext2 and UFS2. The Micro/mini SD card is compatible with 1.0/1.1 software and the Mini/Micro High capacity (SDHD) runs on a 2.0 software - the 2.0 still works with the normal micro SD but the Micro SD high capacity does not work with the 1.0/1.1.