The Secure Digital (SD) card was invented by SanDisk in 2001 and was based on the multi-media card (MMC) standard. Technically, SD is similar to MMC, but adds digital rights management based on CPRM. SD cards also feature a write protection switch, but it is not a hardware feature: the client device has to handle both settings appropriately.
The 2 GB capacity defined by the SD 1.1 standard wasn’t enough as card sizes grew, so the SD 2.0 or SDHC standard was added. It allows for capacities of up to 32 GB today; the standard is potentially ready for capacities of up to 2 TB. SDXC will follow next year, as 32 GB may remain the limit for the SDHC standard. Note that SDHC and SD cards are identical from the outside, so be sure your device supports SDHC before purchasing such a card (4 GB and up).
The first SD cards could be read at 3.6 MB/s and written at 0.8 MB/s. Faster cards were required by the increasing resolutions of digital cameras, as well as more demanding consumers. As a result, SDHC was divided into classes: 2, 4 , 6 ,8 and class 10 ; the numbers represent the minimum sustainable write throughput in MB/s.
So, The class 10 SDHC is the fastest & higest capacities in present days.
It’s not only high resolution digital SLR cameras that require fast memory cards [class 10 sdhc] , allowing them to write several photos per second onto the storage device. Another key application is multi-purpose, high-speed mobile torage, or using these cards as system drives via USB or eSATA card readers.
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