SD or SecureDigital cards were the first-generation of this type of flash memory card. They come in capacities up to 4GB however, there is no guarantee that a 4GB card will work in any SD card socket. According to the standard, 1GB is the maximum guaranteed working capacity. Capacities of 2GB and 4GB require modifications to the standard and flash card readers that will handle that format. Thankfully, a significant number of devices will easily handle cards up to 2GB in capacity.
SDHC stands for SecureDigital – HighCapacity and is an extension to the SD standard that allows for capacities up to 32GB.
In practice, SDHC cards look like standard SD cards however your device must specifically support SDHC cards to use one. All SDHC card readers and devices can also handle standard SD cards as well so it’s what is called “backward compatible”.
You’ll find most SDHC cards start at 4GB capacity.
SDXC stands for SecureDigital – eXtended Capacity and is the newest standard – in fact, it’s so new that these cards are only just now starting to arrive on the market. SDXC cards are again the same shape and size as SDHC and SD cards but offer capacities up to 2048GB. These will be ideal for high-definition (HD) digital video capture at up to 1920×1080-pixel resolution and beyond. For now, SDXC cards will start in 64GB capacities but expect that to grow over time.