Secure Digital High Capacity memory cards, first produced in 2006, look and function almost exactly like regular Secure Digital cards. Like traditional SD cards, SDHC cards work in many types of devices, including cell phones, cameras, tablets and computers. The two differ primarily in storage capacity and compatibility: SD cards work in any device with an SD slot, whereas SDHC cards can hold more data but only work in devices that support the SDHC standard.
SDHC cards hold more data than SD cards. Both types of cards vary in capacity from model to model, but regular SD cards max out at 2GB, while SDHC cards come in sizes between 4GB and 32GB. A 2GB card can hold about 30 minutes of HD video recorded at 9 megabits per second or around 770 10-megapixel photos. A 4GB card can store twice as much, an 8GB card four times as much, and so on.
All electronics that use SD cards -- called "host devices" -- are backward compatible with older versions of SD but not forward compatible. For example, a camera without SDHC support can only use a regular SD card, whereas a model that supports SDHC cards can use both SDHC and regular SD cards. Check a device's manual to find which cards it supports. Even if a device supports SDHC, you might save money by buying a regular SD card for devices that don't use much data, such as basic cell phones or feature phones.
You can't tell SD and SDHC memory cards apart or calculate a card's storage capacity based on physical size. Both types of cards come in three sizes: microSD, miniSD and the original full size. To identify a card's type, read its label or packaging and look for the "HC" mark. Most devices and card readers only support a single card size, unless you use an adapter, so check a device's slot size before buying a card.
Most SDHC cards run at the same speeds as SD cards. Each card has a speed class -- up to 10 -- that indicates its minimum sustained speed in megabytes per second. Some SDHC cards -- but no regular SD cards -- are branded as Ultra High Speed. UHS speed classes, marked with a "U" symbol, indicate one-tenth of the card's speed: An SDHC UHS class 3 card runs at 30 MBps. You can use an SDHC UHS card in a device that doesn't support UHS, but it will run at a slower, non-UHS speed.
Just as SDHC cards offer a higher capacity than SD cards, Secure Digital Extended Capacity cards boost the limit even higher to a theoretical limit of 2TB. SDXC devices support all three types of cards, but devices that support only SD or SDHC can't use an SDXC card. SDXC cards use a different type of formatting that makes them incompatible with older computer systems, such as Windows XP, until you install a compatibility patch.